Bacteria — MCQs

Q: A 45-year-old man is brought to the emergency department after being found down outside of a bar. He does not have any identifying information and is difficult to arouse. On presentation, his temperature is 101.2°F (38.4°C), blood pressure is 109/72 mmHg, pulse is 102/min, and respirations are 18/min. Physical exam reveals an ill-appearing and disheveled man with labored breathing and coughing productive of viscous red sputum. Lung auscultation demonstrates consolidation of the left upper lobe of the patient. Given these findings, cultures are obtained and broad spectrum antibiotics are administered. Which of the following agar types should be used to culture the most likely organism in this case?

MacConkey agar. ***MacConkey agar*** - The patient's presentation with **viscous red "currant jelly" sputum**, **upper lobe consolidation**, and history of **alcohol use** is classic for pneumonia caused by ***Klebsiella pneumoniae***. - **MacConkey agar** is a selective and differential medium used to isolate **gram-negative bacteria** such as *Klebsiella pneumoniae*, which appears as mucoid, lactose-fermenting (pink) colonies. - The "currant jelly sputum" is pathognomonic for *Klebsiella* and distinguishes it from other pneumonias. *Blood agar* - While blood agar is a rich, non-selective medium that supports growth of many organisms including *Streptococcus pneumoniae*, it is not the most appropriate choice for this case. - *Streptococcus pneumoniae* typically causes **rust-colored sputum**, not the viscous red sputum described here. - Though *Klebsiella* can grow on blood agar, **MacConkey agar** is more specific for identifying gram-negative organisms like *Klebsiella*. *Charcoal yeast extract agar* - This medium is specifically designed for the isolation of **Legionella species**, which are fastidious gram-negative rods. - *Legionella* pneumonia typically presents with relative bradycardia, hyponatremia, and diarrhea, not the viscous red sputum characteristic of *Klebsiella*. *Löwenstein-Jensen agar* - This specialized medium is used for the isolation of **mycobacteria**, particularly *Mycobacterium tuberculosis*. - TB typically presents with chronic symptoms (weeks to months), night sweats, and hemoptysis, not the acute presentation with viscous red sputum seen here. *Eaton agar* - This specialized medium is used for the isolation of **Mycoplasma pneumoniae**, which causes "walking pneumonia." - *Mycoplasma* pneumonia is typically mild with dry cough and patchy infiltrates, not the severe lobar consolidation and viscous red sputum seen in this case.

Q: A 40-year-old man presents to a community health center for a routine check-up. The medical history is significant for a major depressive disorder that began around the time he arrived in the United States from India, his native country. For the last few months, he has been living in the local homeless shelter and also reports being incarcerated for an extended period of time. The patient has smoked 1 pack of cigarettes daily for the last 20 years. The vital signs include the following: the heart rate is 68/min, the respiratory rate is 18/min, the temperature is 37.1°C (98.8°F), and the blood pressure is 130/88 mm Hg. He appears unkempt and speaks in a monotone. Coarse breath sounds are auscultated in the lung bases bilaterally. Which of the following is recommended for this patient?

Quantiferon testing. ***Quantiferon testing*** - This patient has multiple risk factors for **tuberculosis (TB)**, including being born in a **TB-endemic country (India)**, **homelessness**, prior **incarceration**, and a history of **major depressive disorder** (which can lead to poor self-care and increased exposure risk). - **Interferon-gamma release assays (IGRAs)** like Quantiferon testing are preferred over the tuberculin skin test (TST) in individuals who have received the **BCG vaccine** (common in India) since BCG can cause false positives with TSTs. *Tuberculin skin test* - While a TST screens for latent TB, its utility in this patient is limited due to his origin from **India**, where widespread **BCG vaccination** often leads to **false-positive results**. - **IGRAs** are generally preferred in individuals vaccinated with BCG as they are not affected by prior BCG vaccination. *Low-dose computerized tomography (CT) Scan* - **Low-dose CT (LDCT)** is primarily used for **lung cancer screening** in high-risk individuals, typically heavy smokers aged 50-80 years. - This patient is 40 years old, which is outside the recommended age range for routine LDCT screening for lung cancer. *Chest X-ray* - A **chest X-ray** is typically used to evaluate symptoms like cough, dyspnea, or to follow up on abnormal findings, and can identify active pulmonary TB. - However, in this asymptomatic patient, a chest X-ray is not the initial recommended screening test for **latent TB infection**, especially given the lack of specific respiratory symptoms beyond coarse breath sounds. *Pulmonary function test* - **Pulmonary function tests (PFTs)** are used to diagnose and assess the severity of lung diseases like **asthma** or **COPD**. - This patient does not present with symptoms that would necessitate PFTs as a routine screening measure or as the initial diagnostic step for his risk factors.

Q: A 28-year-old man presents to the office complaining of a sore throat, difficulty swallowing, and difficulty opening his mouth for the past 5 days. He states that he had symptoms like this before and "was given some antibiotics that made him feel better". He is up to date on his immunizations. On examination, his temperature is 39.5°C (103.2°F) and he has bilateral cervical lymphadenopathy. An oropharyngeal exam is difficult, because the patient finds it painful to fully open his mouth. However, you are able to view an erythematous pharynx as well as a large, unilateral lesion superior to the left tonsil. A rapid antigen detection test is negative. Based on this clinical presentation, what is a serious complication of the most likely diagnosis?

Lemierre syndrome. ***Lemierre syndrome*** - The patient's symptoms (sore throat, dysphagia, trismus), fever, unilateral peritonsillar lesion, and cervical lymphadenopathy are highly suggestive of a **peritonsillar abscess**. - **Lemierre syndrome** is a severe complication of oropharyngeal infections, especially peritonsillar abscesses, involving thrombophlebitis of the internal jugular vein and septic emboli. *Infectious mononucleosis* - While mononucleosis can cause severe pharyngitis and lymphadenopathy, it typically presents with **bilateral tonsillar enlargement** and exudates, not a unilateral peritonsillar lesion. - Furthermore, a history of recurrent strep-like symptoms and effective antibiotic response makes streptococcal infection (and its complications) more likely. *Acute rheumatic fever* - This is a non-suppurative complication of untreated **Group A Streptococcus (GAS)** pharyngitis. - The patient's unilateral peritonsillar lesion points to a localized suppurative infection (abscess), making acute rheumatic fever less likely as a direct complication of the *current* presentation. *Whooping cough* - Also known as **pertussis**, this is a respiratory infection characterized by severe, **paroxysmal coughing fits** followed by a characteristic "whoop." - The patient's symptoms of sore throat, difficulty swallowing, and a peritonsillar lesion are inconsistent with whooping cough. *Diphtheria* - Diphtheria causes a severe sore throat and the formation of a **thick, grey pseudomembrane** on the tonsils and pharynx. - While serious, the patient's symptoms and the absence of a pseudomembrane make diphtheria less likely, especially with an updated immunization status.

Q: Which of the following events is likely to occur in the germinal center?

Isotype switching. ***Isotype switching*** - **Isotype switching**, also known as class-switch recombination, is a hallmark event in the **germinal center**, allowing B cells to change the heavy chain constant region of their antibodies, thereby altering their effector functions while retaining antigen specificity. - This process is crucial for generating diversity in antibody responses, enabling the production of antibodies like **IgG, IgA, or IgE** from an initial IgM response, depending on the immunological need. *T-cell negative selection* - **Negative selection** of T cells occurs in the **thymus**, where T cells that bind too strongly to self-peptide-MHC complexes are eliminated to prevent autoimmunity. - This process is part of T-cell maturation and is distinct from events that occur in secondary lymphoid organs like the germinal center. *Formation of double-positive T cells* - **Double-positive T cells** (CD4+ CD8+) are an intermediate stage in **T-cell development** that occurs exclusively in the **cortex of the thymus**. - These cells undergo both positive and negative selection to mature into single-positive CD4+ or CD8+ T cells. *Development of early pro-B cells* - The development of **early pro-B cells** is an initial stage of **B-cell lymphopoiesis** that occurs in the **bone marrow**, not in the germinal center. - During this stage, B-cell precursors rearrange their **heavy chain variable region (V(D)J)** genes. *Development of immature B cells* - The development of **immature B cells** also takes place in the **bone marrow**, where they complete heavy and light chain rearrangement and express IgM on their surface. - These immature B cells then migrate to peripheral lymphoid organs to complete their maturation, but this initial development is separate from germinal center reactions.

Q: A previously healthy 24-year-old woman comes to the physician because of a 1-day history of nausea and weakness. She is sexually active with 2 male partners and uses an oral contraceptive; she uses condoms inconsistently. Her last menstrual period was 4 days ago. Her temperature is 38.4°C (101°F). Physical examination shows right costovertebral angle tenderness. Pelvic examination is normal. Which of the following is the most likely cause of this patient's condition?

Ascending bacteria from the bladder. ***Ascending bacteria from the bladder*** - The patient presents with **fever**, **nausea**, **weakness**, and **right costovertebral angle (CVA) tenderness**, which are classic symptoms of **acute pyelonephritis**. - **Pyelonephritis** most commonly results from an **ascending urinary tract infection**, where bacteria (typically *E. coli*) from the bladder travel up the ureters to infect the kidneys. - This accounts for approximately **95% of pyelonephritis cases** in young women. *Noninfectious inflammation of the bladder* - **Noninfectious cystitis** (interstitial cystitis) would not typically present with systemic symptoms like **fever** and **nausea**, or with **CVA tenderness**, which indicates kidney involvement. - Bladder inflammation typically causes dysuria and frequency without systemic signs of infection. *Ascending bacteria from the endocervix* - **Ascending bacteria from the endocervix** can cause **pelvic inflammatory disease (PID)**, which presents with lower abdominal pain, cervical motion tenderness, and vaginal discharge. - While PID can cause fever, the **normal pelvic examination** in this patient rules out this diagnosis, and PID **does not typically cause CVA tenderness**. *Decreased renal calcium reabsorption* - **Decreased renal calcium reabsorption** is associated with **hypercalciuria** and **nephrolithiasis** (kidney stones), which can present with acute flank pain if obstruction occurs. - However, this condition does not explain the **fever** and systemic symptoms characteristic of an acute infectious process. *Decreased urinary pH* - **Decreased urinary pH** (acidic urine) can predispose to certain types of kidney stone formation but is not a direct cause of **pyelonephritis**. - It does not explain the presence of **fever**, **CVA tenderness**, and systemic symptoms indicative of a bacterial kidney infection.

Q: A 2-day-old boy born to a primigravida with no complications has an ear infection. He is treated with antibiotics and sent home. His parents bring him back 1 month later with an erythematous and swollen umbilical cord still attached to the umbilicus. A complete blood cell count shows the following: Hemoglobin 18.1 g/dL Hematocrit 43.7% Leukocyte count 13,000/mm3 Neutrophils 85% Lymphocytes 10% Monocytes 5% Platelet count 170,000/mm3 The immunoglobulin levels are normal. The absence or deficiency of which of the following most likely led to this patient’s condition?

CD18. ***CD18*** - The clinical presentation of **delayed umbilical cord separation** (classic hallmark), recurrent infections (ear infection), and **leukocytosis with neutrophilia** strongly suggests **leukocyte adhesion deficiency type 1** (LAD-1). - LAD-1 is caused by a defect in the **CD18** subunit (β2 integrin), which combines with CD11a, CD11b, or CD11c to form the **β2 integrin complex** (LFA-1, Mac-1, CR3/CR4) crucial for leukocyte adhesion to endothelium and extravasation into sites of infection. - Without functional CD18, neutrophils cannot migrate to infection sites despite elevated counts in circulation. *Prostaglandin E2* - **Prostaglandin E2** is a lipid mediator involved in inflammation, fever, and pain, but its deficiency would not directly cause a defect in leukocyte adhesion or delayed umbilical cord separation. - It plays a role in vasodilation and preventing platelet aggregation, unrelated to the described immune defect. *Histamine* - **Histamine** is a vasoactive amine released by mast cells and basophils, primarily involved in allergic reactions and gastric acid secretion. - A deficiency in histamine would not lead to delayed umbilical cord separation or impaired leukocyte adhesion. *TNF* - **Tumor Necrosis Factor (TNF)** is a pro-inflammatory cytokine essential for host defense against infection and immune regulation. - While a deficiency could lead to increased susceptibility to certain infections, it does not directly impair leukocyte adhesion or cause delayed umbilical cord separation. *IL-1* - **Interleukin-1 (IL-1)** is a pro-inflammatory cytokine with functions similar to TNF, playing a key role in the acute phase response and immune activation. - A deficiency would impair inflammatory responses but would not specifically cause the adhesion defect and delayed umbilical cord separation seen in this patient.

Q: An 18-year-old college freshman scrapes his knee after falling from his bike. He applies some topical neomycin because he knows that it has antibiotic properties. As he is also in biology class, he decides to research the mechanism of action of neomycin and finds that it interferes with formation of the 30S initiation complex in bacteria. What is the messenger RNA (mRNA) signal recognized by the 30S ribosomal subunit necessary for the initiation of translation?

Shine-Dalgarno sequence. ***Shine-Dalgarno sequence*** - The **Shine-Dalgarno sequence** is a purine-rich sequence in bacterial mRNA that is complementary to a sequence in the 16S rRNA of the 30S ribosomal subunit. - This interaction correctly positions the **30S ribosomal subunit** on the mRNA, allowing it to find the start codon and initiate translation. *UAA, UAG, and UGA codons* - These are **stop codons** that signal the termination of translation rather than its initiation. - They are recognized by release factors, not the 30S ribosomal subunit, during the elongation phase of protein synthesis. *Kozak sequence* - The **Kozak sequence** is a sequence in eukaryotic mRNA that helps the eukaryotic 40S ribosomal subunit identify the start codon (usually AUG). - It plays an analogous role to the Shine-Dalgarno sequence but functions in **eukaryotic translation initiation**, not prokaryotic. *Polyadenosine tail* - The **poly(A) tail** is a long stretch of adenosine ribonucleotides added to the 3' end of eukaryotic mRNA. - It is involved in mRNA stability, export from the nucleus, and translation efficiency in **eukaryotes**, but it is not a direct signal for ribosomal subunit binding for initiation in prokaryotes. *5' methyl-guanosine cap* - The **5' methyl-guanosine cap** is a modified guanosine nucleotide added to the 5' end of eukaryotic mRNA. - It is crucial for mRNA stability, nuclear export, and ribosome binding to initiate translation in **eukaryotes**, not prokaryotes.

Q: A 55-year-old man with type 2 diabetes mellitus comes to the physician because of a 4-day history of fever, chills, nausea, and abdominal pain. He does not use illicit drugs. His temperature is 39°C (102.2°F). Physical examination shows right upper quadrant tenderness. Ultrasonography of the abdomen shows a 6-cm solitary, fluid-filled cavity in the right hepatic lobe. CT-guided percutaneous aspiration of the cavity produces yellowish-green fluid. Culture of the aspirated fluid grows gram-negative, lactose-fermenting rods. Which of the following is the most likely cause of the color of the aspirated fluid?

Myeloperoxidase. ***Myeloperoxidase*** - The yellowish-green color of the aspirated fluid, combined with the presence of **gram-negative, lactose-fermenting rods** (suggesting an enteric bacterial infection), indicates a **pyogenic liver abscess**. - **Myeloperoxidase** is an enzyme found in **neutrophils**, which are abundant in pus and contribute to its characteristic yellowish-green hue. *Biliverdin* - **Biliverdin** is a green pigment formed from the breakdown of **heme**, typically associated with jaundice or bile-containing fluids. - While bile might be present if the abscess communicated with the biliary tree, the primary cause of a pus's specific yellowish-green color in a bacterial infection is not biliverdin. *Prodigiosin* - **Prodigiosin** is a distinctive **red pigment** produced by certain bacterial species, most notably *Serratia marcescens*. - This pigment would result in a red or pink discoloration, not the yellowish-green seen in this patient's aspirate. *Staphyloxanthin* - **Staphyloxanthin** is a **golden-yellow pigment** produced by *Staphylococcus aureus*, contributing to its characteristic colony color. - While *S. aureus* can cause abscesses, the culture here grew **gram-negative, lactose-fermenting rods**, making staphyloxanthin an unlikely cause for the color. *Pyoverdine* - **Pyoverdine** is a **fluorescent yellow-green pigment** produced by *Pseudomonas aeruginosa*. - Although it produces a greenish hue, *Pseudomonas aeruginosa* is a **non-lactose-fermenting gram-negative rod**, which contradicts the culture results of lactose-fermenting organisms.

Q: An 87-year-old woman presents with fever, fatigue, and blood in her urine. She says that symptoms onset 3 days ago and have not improved. She describes the fatigue as severe and that her urine also has an odd smell to it. She denies any recent history of chills, abdominal or flank pain, or similar past symptoms. Past medical history is significant for a urinary tract infection (UTI) diagnosed 2 weeks ago for which she just completed a course of oral antibiotics. The vitals signs include pulse rate 87/min and temperature 38.8°C (101.8°F). Physical examination is unremarkable. Urinalysis reveals the presence of acid-fast bacilli. The patient is admitted and an appropriate antibiotic regimen is started. Which of the following would be the best test to screen for latent infection by the microorganism most likely responsible for this patient’s condition?

Interferon-gamma release assays. ***Interferon-gamma release assays*** - This assay directly screens for a **T-cell mediated immune response** to *Mycobacterium tuberculosis* antigens, indicating either latent or active infection. - Given the presence of **acid-fast bacilli** in urine and a history suggestive of **renal tuberculosis** (UTI-like symptoms despite antibiotic treatment, fever, fatigue), this test is highly appropriate for detecting latent infection with the causative organism. *Culture in Löwenstein-Jensen media* - This is a culture method used to **isolate and grow mycobacteria** from a sample, which would confirm active infection rather than screen for latent disease. - While it could be used for diagnosis, it's not the best test for *screening for latent infection*. *Chest X-ray* - A chest X-ray is used to detect **pulmonary tuberculosis**, which is the most common form of active TB. - It would not screen for **latent tuberculosis infection** itself or specifically for **extrapulmonary forms** like renal tuberculosis, which this patient likely has. *Gram stain of urine sample* - A Gram stain is effective for identifying common **bacterial pathogens** (Gram-positive or Gram-negative) in urine. - **Mycobacteria** are acid-fast and do **not stain well with Gram stain**, making this test unsuitable for their detection or for screening latent TB. *Sputum culture* - **Sputum culture** is used to diagnose **pulmonary tuberculosis** by detecting *Mycobacterium tuberculosis* in respiratory secretions. - The patient's symptoms are localized to the urinary tract with no respiratory complaints, making sputum culture irrelevant for this presentation.

A 45-year-old man is brought to the emergency department after being found down outside of a bar. He does not have any identifying information and is difficult to arouse. On presentation, his temperature is 101.2°F (38.4°C), blood pressure is 109/72 mmHg, pulse is 102/min, and respirations are 18/min. Physical exam reveals an ill-appearing and disheveled man with labored breathing and coughing productive of viscous red sputum. Lung auscultation demonstrates consolidation of the left upper lobe of the patient. Given these findings, cultures are obtained and broad spectrum antibiotics are administered. Which of the following agar types should be used to culture the most likely organism in this case?

A 9-year-old girl comes to the clinic with a chief complaint of a swollen eye and sinus infection for 4 days. She complained of left nasal pain prior to these symptoms. The patient noticed that the swelling and redness of her left eye has progressively worsened. It has been difficult to open her eyelids, and she complains of diplopia and pain during ocular movement. The visual acuity is 20/20 in both eyes. Intraocular pressure measurement shows values of 23 and 14 mm Hg in the right and left eyes, respectively. The test results for the complete blood count, ESR, and CRP are as follows (on admission): CBC results Leukocytes 18,000 cells/mm3 Neutrophils 80% Lymphocytes 14% Eosinophils 1% Basophils 0% Monocytes 5% Hemoglobin 12 g/dL ESR 65 CRP 4.6 The organism causing the above condition is destroyed by which one of the following immunological processes?

A 40-year-old man presents to a community health center for a routine check-up. The medical history is significant for a major depressive disorder that began around the time he arrived in the United States from India, his native country. For the last few months, he has been living in the local homeless shelter and also reports being incarcerated for an extended period of time. The patient has smoked 1 pack of cigarettes daily for the last 20 years. The vital signs include the following: the heart rate is 68/min, the respiratory rate is 18/min, the temperature is 37.1°C (98.8°F), and the blood pressure is 130/88 mm Hg. He appears unkempt and speaks in a monotone. Coarse breath sounds are auscultated in the lung bases bilaterally. Which of the following is recommended for this patient?

A 28-year-old man presents to the office complaining of a sore throat, difficulty swallowing, and difficulty opening his mouth for the past 5 days. He states that he had symptoms like this before and "was given some antibiotics that made him feel better". He is up to date on his immunizations. On examination, his temperature is 39.5°C (103.2°F) and he has bilateral cervical lymphadenopathy. An oropharyngeal exam is difficult, because the patient finds it painful to fully open his mouth. However, you are able to view an erythematous pharynx as well as a large, unilateral lesion superior to the left tonsil. A rapid antigen detection test is negative. Based on this clinical presentation, what is a serious complication of the most likely diagnosis?

Which of the following events is likely to occur in the germinal center?

A previously healthy 24-year-old woman comes to the physician because of a 1-day history of nausea and weakness. She is sexually active with 2 male partners and uses an oral contraceptive; she uses condoms inconsistently. Her last menstrual period was 4 days ago. Her temperature is 38.4°C (101°F). Physical examination shows right costovertebral angle tenderness. Pelvic examination is normal. Which of the following is the most likely cause of this patient's condition?

A 2-day-old boy born to a primigravida with no complications has an ear infection. He is treated with antibiotics and sent home. His parents bring him back 1 month later with an erythematous and swollen umbilical cord still attached to the umbilicus. A complete blood cell count shows the following: Hemoglobin 18.1 g/dL Hematocrit 43.7% Leukocyte count 13,000/mm3 Neutrophils 85% Lymphocytes 10% Monocytes 5% Platelet count 170,000/mm3 The immunoglobulin levels are normal. The absence or deficiency of which of the following most likely led to this patient’s condition?

An 18-year-old college freshman scrapes his knee after falling from his bike. He applies some topical neomycin because he knows that it has antibiotic properties. As he is also in biology class, he decides to research the mechanism of action of neomycin and finds that it interferes with formation of the 30S initiation complex in bacteria. What is the messenger RNA (mRNA) signal recognized by the 30S ribosomal subunit necessary for the initiation of translation?

A 55-year-old man with type 2 diabetes mellitus comes to the physician because of a 4-day history of fever, chills, nausea, and abdominal pain. He does not use illicit drugs. His temperature is 39°C (102.2°F). Physical examination shows right upper quadrant tenderness. Ultrasonography of the abdomen shows a 6-cm solitary, fluid-filled cavity in the right hepatic lobe. CT-guided percutaneous aspiration of the cavity produces yellowish-green fluid. Culture of the aspirated fluid grows gram-negative, lactose-fermenting rods. Which of the following is the most likely cause of the color of the aspirated fluid?

Q10

An 87-year-old woman presents with fever, fatigue, and blood in her urine. She says that symptoms onset 3 days ago and have not improved. She describes the fatigue as severe and that her urine also has an odd smell to it. She denies any recent history of chills, abdominal or flank pain, or similar past symptoms. Past medical history is significant for a urinary tract infection (UTI) diagnosed 2 weeks ago for which she just completed a course of oral antibiotics. The vitals signs include pulse rate 87/min and temperature 38.8°C (101.8°F). Physical examination is unremarkable. Urinalysis reveals the presence of acid-fast bacilli. The patient is admitted and an appropriate antibiotic regimen is started. Which of the following would be the best test to screen for latent infection by the microorganism most likely responsible for this patient’s condition?

Bacteria US Medical PG Practice Questions and MCQs

Question 1: A 45-year-old man is brought to the emergency department after being found down outside of a bar. He does not have any identifying information and is difficult to arouse. On presentation, his temperature is 101.2°F (38.4°C), blood pressure is 109/72 mmHg, pulse is 102/min, and respirations are 18/min. Physical exam reveals an ill-appearing and disheveled man with labored breathing and coughing productive of viscous red sputum. Lung auscultation demonstrates consolidation of the left upper lobe of the patient. Given these findings, cultures are obtained and broad spectrum antibiotics are administered. Which of the following agar types should be used to culture the most likely organism in this case?

A. Charcoal yeast extract agar
B. Blood agar
C. MacConkey agar (Correct Answer)
D. Eaton agar
E. Löwenstein-Jensen agar

Explanation: ***MacConkey agar*** - The patient's presentation with **viscous red "currant jelly" sputum**, **upper lobe consolidation**, and history of **alcohol use** is classic for pneumonia caused by ***Klebsiella pneumoniae***. - **MacConkey agar** is a selective and differential medium used to isolate **gram-negative bacteria** such as *Klebsiella pneumoniae*, which appears as mucoid, lactose-fermenting (pink) colonies. - The "currant jelly sputum" is pathognomonic for *Klebsiella* and distinguishes it from other pneumonias. *Blood agar* - While blood agar is a rich, non-selective medium that supports growth of many organisms including *Streptococcus pneumoniae*, it is not the most appropriate choice for this case. - *Streptococcus pneumoniae* typically causes **rust-colored sputum**, not the viscous red sputum described here. - Though *Klebsiella* can grow on blood agar, **MacConkey agar** is more specific for identifying gram-negative organisms like *Klebsiella*. *Charcoal yeast extract agar* - This medium is specifically designed for the isolation of **Legionella species**, which are fastidious gram-negative rods. - *Legionella* pneumonia typically presents with relative bradycardia, hyponatremia, and diarrhea, not the viscous red sputum characteristic of *Klebsiella*. *Löwenstein-Jensen agar* - This specialized medium is used for the isolation of **mycobacteria**, particularly *Mycobacterium tuberculosis*. - TB typically presents with chronic symptoms (weeks to months), night sweats, and hemoptysis, not the acute presentation with viscous red sputum seen here. *Eaton agar* - This specialized medium is used for the isolation of **Mycoplasma pneumoniae**, which causes "walking pneumonia." - *Mycoplasma* pneumonia is typically mild with dry cough and patchy infiltrates, not the severe lobar consolidation and viscous red sputum seen in this case.

Question 2: A 9-year-old girl comes to the clinic with a chief complaint of a swollen eye and sinus infection for 4 days. She complained of left nasal pain prior to these symptoms. The patient noticed that the swelling and redness of her left eye has progressively worsened. It has been difficult to open her eyelids, and she complains of diplopia and pain during ocular movement. The visual acuity is 20/20 in both eyes. Intraocular pressure measurement shows values of 23 and 14 mm Hg in the right and left eyes, respectively. The test results for the complete blood count, ESR, and CRP are as follows (on admission): CBC results Leukocytes 18,000 cells/mm3 Neutrophils 80% Lymphocytes 14% Eosinophils 1% Basophils 0% Monocytes 5% Hemoglobin 12 g/dL ESR 65 CRP 4.6 The organism causing the above condition is destroyed by which one of the following immunological processes?

A. Perforins and granzymes by natural killer cells
B. Release of cytotoxic granules by cytotoxic T cells
C. Phagolysosome formation by neutrophils (Correct Answer)
D. Activation of cytosolic caspases
E. Complement-mediated membrane attack complex formation

Explanation: ***Phagolysosome formation by neutrophils*** - The patient presents with symptoms highly suggestive of **orbital cellulitis**, evidenced by a swollen eye, pain with ocular movement, diplopia, and a history of sinus infection. The laboratory findings, including **leukocytosis (18,000 cells/mm3)** with a high percentage of **neutrophils (80%)**, and elevated **ESR (65)** and **CRP (4.6)**, indicate an active **bacterial infection**. - **Neutrophils** are the primary immune cells responding to bacterial infections. Their main mechanism of destroying bacteria involves **phagocytosis**, where they engulf the pathogen and fuse the phagosome with lysosomes to form a **phagolysosome**, leading to bacterial degradation through oxidative burst and enzymatic digestion. *Perforins and granzymes by natural killer cells* - **Natural killer (NK) cells** primarily target **virus-infected cells** and **tumor cells** by inducing apoptosis through the release of perforins and granzymes. - This mechanism is not the primary way the immune system combats **bacterial infections** like the orbital cellulitis described. *Release of cytotoxic granules by cytotoxic T cells* - **Cytotoxic T lymphocytes (CTLs)**, or CD8+ T cells, are crucial for eliminating **intracellular pathogens (e.g., viruses)** and **cancer cells** by inducing apoptosis. - While important for cell-mediated immunity, CTLs are not the main effector cells against the **extracellular bacteria** commonly causing orbital cellulitis. *Activation of cytosolic caspases* - **Caspases** are a family of proteases that play an essential role in programmed cell death, or **apoptosis**. - While apoptosis is a part of immune regulation, the **direct destruction of extracellular bacterial pathogens** in an active infection is not mediated by the activation of cytosolic caspases within the pathogen or host cells as a primary defense mechanism. *Complement-mediated membrane attack complex formation* - The **membrane attack complex (MAC)** formed by complement proteins C5b-C9 creates pores in bacterial membranes, leading to lysis. - While MAC is effective against some gram-negative bacteria, the encapsulated bacteria commonly causing orbital cellulitis (such as **Streptococcus pneumoniae** and **Staphylococcus aureus**) are relatively **resistant to MAC-mediated lysis** due to their thick peptidoglycan layer or capsules. The primary mechanism of destruction for these pathogens is **neutrophil-mediated phagocytosis** rather than complement-mediated lysis.

Question 3: A 40-year-old man presents to a community health center for a routine check-up. The medical history is significant for a major depressive disorder that began around the time he arrived in the United States from India, his native country. For the last few months, he has been living in the local homeless shelter and also reports being incarcerated for an extended period of time. The patient has smoked 1 pack of cigarettes daily for the last 20 years. The vital signs include the following: the heart rate is 68/min, the respiratory rate is 18/min, the temperature is 37.1°C (98.8°F), and the blood pressure is 130/88 mm Hg. He appears unkempt and speaks in a monotone. Coarse breath sounds are auscultated in the lung bases bilaterally. Which of the following is recommended for this patient?

A. Quantiferon testing (Correct Answer)
B. Tuberculin skin test
C. Low-dose computerized tomography (CT) Scan
D. Chest X-ray
E. Pulmonary function test

Explanation: ***Quantiferon testing*** - This patient has multiple risk factors for **tuberculosis (TB)**, including being born in a **TB-endemic country (India)**, **homelessness**, prior **incarceration**, and a history of **major depressive disorder** (which can lead to poor self-care and increased exposure risk). - **Interferon-gamma release assays (IGRAs)** like Quantiferon testing are preferred over the tuberculin skin test (TST) in individuals who have received the **BCG vaccine** (common in India) since BCG can cause false positives with TSTs. *Tuberculin skin test* - While a TST screens for latent TB, its utility in this patient is limited due to his origin from **India**, where widespread **BCG vaccination** often leads to **false-positive results**. - **IGRAs** are generally preferred in individuals vaccinated with BCG as they are not affected by prior BCG vaccination. *Low-dose computerized tomography (CT) Scan* - **Low-dose CT (LDCT)** is primarily used for **lung cancer screening** in high-risk individuals, typically heavy smokers aged 50-80 years. - This patient is 40 years old, which is outside the recommended age range for routine LDCT screening for lung cancer. *Chest X-ray* - A **chest X-ray** is typically used to evaluate symptoms like cough, dyspnea, or to follow up on abnormal findings, and can identify active pulmonary TB. - However, in this asymptomatic patient, a chest X-ray is not the initial recommended screening test for **latent TB infection**, especially given the lack of specific respiratory symptoms beyond coarse breath sounds. *Pulmonary function test* - **Pulmonary function tests (PFTs)** are used to diagnose and assess the severity of lung diseases like **asthma** or **COPD**. - This patient does not present with symptoms that would necessitate PFTs as a routine screening measure or as the initial diagnostic step for his risk factors.

Question 4: A 28-year-old man presents to the office complaining of a sore throat, difficulty swallowing, and difficulty opening his mouth for the past 5 days. He states that he had symptoms like this before and "was given some antibiotics that made him feel better". He is up to date on his immunizations. On examination, his temperature is 39.5°C (103.2°F) and he has bilateral cervical lymphadenopathy. An oropharyngeal exam is difficult, because the patient finds it painful to fully open his mouth. However, you are able to view an erythematous pharynx as well as a large, unilateral lesion superior to the left tonsil. A rapid antigen detection test is negative. Based on this clinical presentation, what is a serious complication of the most likely diagnosis?

A. Infectious mononucleosis
B. Acute rheumatic fever
C. Lemierre syndrome (Correct Answer)
D. Whooping cough
E. Diphtheria

Explanation: ***Lemierre syndrome*** - The patient's symptoms (sore throat, dysphagia, trismus), fever, unilateral peritonsillar lesion, and cervical lymphadenopathy are highly suggestive of a **peritonsillar abscess**. - **Lemierre syndrome** is a severe complication of oropharyngeal infections, especially peritonsillar abscesses, involving thrombophlebitis of the internal jugular vein and septic emboli. *Infectious mononucleosis* - While mononucleosis can cause severe pharyngitis and lymphadenopathy, it typically presents with **bilateral tonsillar enlargement** and exudates, not a unilateral peritonsillar lesion. - Furthermore, a history of recurrent strep-like symptoms and effective antibiotic response makes streptococcal infection (and its complications) more likely. *Acute rheumatic fever* - This is a non-suppurative complication of untreated **Group A Streptococcus (GAS)** pharyngitis. - The patient's unilateral peritonsillar lesion points to a localized suppurative infection (abscess), making acute rheumatic fever less likely as a direct complication of the *current* presentation. *Whooping cough* - Also known as **pertussis**, this is a respiratory infection characterized by severe, **paroxysmal coughing fits** followed by a characteristic "whoop." - The patient's symptoms of sore throat, difficulty swallowing, and a peritonsillar lesion are inconsistent with whooping cough. *Diphtheria* - Diphtheria causes a severe sore throat and the formation of a **thick, grey pseudomembrane** on the tonsils and pharynx. - While serious, the patient's symptoms and the absence of a pseudomembrane make diphtheria less likely, especially with an updated immunization status.

Question 5: Which of the following events is likely to occur in the germinal center?

A. T-cell negative selection
B. Formation of double-positive T cells
C. Development of early pro-B cells
D. Isotype switching (Correct Answer)
E. Development of immature B cells

Explanation: ***Isotype switching*** - **Isotype switching**, also known as class-switch recombination, is a hallmark event in the **germinal center**, allowing B cells to change the heavy chain constant region of their antibodies, thereby altering their effector functions while retaining antigen specificity. - This process is crucial for generating diversity in antibody responses, enabling the production of antibodies like **IgG, IgA, or IgE** from an initial IgM response, depending on the immunological need. *T-cell negative selection* - **Negative selection** of T cells occurs in the **thymus**, where T cells that bind too strongly to self-peptide-MHC complexes are eliminated to prevent autoimmunity. - This process is part of T-cell maturation and is distinct from events that occur in secondary lymphoid organs like the germinal center. *Formation of double-positive T cells* - **Double-positive T cells** (CD4+ CD8+) are an intermediate stage in **T-cell development** that occurs exclusively in the **cortex of the thymus**. - These cells undergo both positive and negative selection to mature into single-positive CD4+ or CD8+ T cells. *Development of early pro-B cells* - The development of **early pro-B cells** is an initial stage of **B-cell lymphopoiesis** that occurs in the **bone marrow**, not in the germinal center. - During this stage, B-cell precursors rearrange their **heavy chain variable region (V(D)J)** genes. *Development of immature B cells* - The development of **immature B cells** also takes place in the **bone marrow**, where they complete heavy and light chain rearrangement and express IgM on their surface. - These immature B cells then migrate to peripheral lymphoid organs to complete their maturation, but this initial development is separate from germinal center reactions.

Question 6: A previously healthy 24-year-old woman comes to the physician because of a 1-day history of nausea and weakness. She is sexually active with 2 male partners and uses an oral contraceptive; she uses condoms inconsistently. Her last menstrual period was 4 days ago. Her temperature is 38.4°C (101°F). Physical examination shows right costovertebral angle tenderness. Pelvic examination is normal. Which of the following is the most likely cause of this patient's condition?

A. Noninfectious inflammation of the bladder
B. Ascending bacteria from the endocervix
C. Decreased renal calcium reabsorption
D. Decreased urinary pH
E. Ascending bacteria from the bladder (Correct Answer)

Explanation: ***Ascending bacteria from the bladder*** - The patient presents with **fever**, **nausea**, **weakness**, and **right costovertebral angle (CVA) tenderness**, which are classic symptoms of **acute pyelonephritis**. - **Pyelonephritis** most commonly results from an **ascending urinary tract infection**, where bacteria (typically *E. coli*) from the bladder travel up the ureters to infect the kidneys. - This accounts for approximately **95% of pyelonephritis cases** in young women. *Noninfectious inflammation of the bladder* - **Noninfectious cystitis** (interstitial cystitis) would not typically present with systemic symptoms like **fever** and **nausea**, or with **CVA tenderness**, which indicates kidney involvement. - Bladder inflammation typically causes dysuria and frequency without systemic signs of infection. *Ascending bacteria from the endocervix* - **Ascending bacteria from the endocervix** can cause **pelvic inflammatory disease (PID)**, which presents with lower abdominal pain, cervical motion tenderness, and vaginal discharge. - While PID can cause fever, the **normal pelvic examination** in this patient rules out this diagnosis, and PID **does not typically cause CVA tenderness**. *Decreased renal calcium reabsorption* - **Decreased renal calcium reabsorption** is associated with **hypercalciuria** and **nephrolithiasis** (kidney stones), which can present with acute flank pain if obstruction occurs. - However, this condition does not explain the **fever** and systemic symptoms characteristic of an acute infectious process. *Decreased urinary pH* - **Decreased urinary pH** (acidic urine) can predispose to certain types of kidney stone formation but is not a direct cause of **pyelonephritis**. - It does not explain the presence of **fever**, **CVA tenderness**, and systemic symptoms indicative of a bacterial kidney infection.

Question 7: A 2-day-old boy born to a primigravida with no complications has an ear infection. He is treated with antibiotics and sent home. His parents bring him back 1 month later with an erythematous and swollen umbilical cord still attached to the umbilicus. A complete blood cell count shows the following: Hemoglobin 18.1 g/dL Hematocrit 43.7% Leukocyte count 13,000/mm3 Neutrophils 85% Lymphocytes 10% Monocytes 5% Platelet count 170,000/mm3 The immunoglobulin levels are normal. The absence or deficiency of which of the following most likely led to this patient’s condition?

A. Prostaglandin E2
B. Histamine
C. CD18 (Correct Answer)
D. TNF
E. IL-1

Explanation: ***CD18*** - The clinical presentation of **delayed umbilical cord separation** (classic hallmark), recurrent infections (ear infection), and **leukocytosis with neutrophilia** strongly suggests **leukocyte adhesion deficiency type 1** (LAD-1). - LAD-1 is caused by a defect in the **CD18** subunit (β2 integrin), which combines with CD11a, CD11b, or CD11c to form the **β2 integrin complex** (LFA-1, Mac-1, CR3/CR4) crucial for leukocyte adhesion to endothelium and extravasation into sites of infection. - Without functional CD18, neutrophils cannot migrate to infection sites despite elevated counts in circulation. *Prostaglandin E2* - **Prostaglandin E2** is a lipid mediator involved in inflammation, fever, and pain, but its deficiency would not directly cause a defect in leukocyte adhesion or delayed umbilical cord separation. - It plays a role in vasodilation and preventing platelet aggregation, unrelated to the described immune defect. *Histamine* - **Histamine** is a vasoactive amine released by mast cells and basophils, primarily involved in allergic reactions and gastric acid secretion. - A deficiency in histamine would not lead to delayed umbilical cord separation or impaired leukocyte adhesion. *TNF* - **Tumor Necrosis Factor (TNF)** is a pro-inflammatory cytokine essential for host defense against infection and immune regulation. - While a deficiency could lead to increased susceptibility to certain infections, it does not directly impair leukocyte adhesion or cause delayed umbilical cord separation. *IL-1* - **Interleukin-1 (IL-1)** is a pro-inflammatory cytokine with functions similar to TNF, playing a key role in the acute phase response and immune activation. - A deficiency would impair inflammatory responses but would not specifically cause the adhesion defect and delayed umbilical cord separation seen in this patient.

Question 8: An 18-year-old college freshman scrapes his knee after falling from his bike. He applies some topical neomycin because he knows that it has antibiotic properties. As he is also in biology class, he decides to research the mechanism of action of neomycin and finds that it interferes with formation of the 30S initiation complex in bacteria. What is the messenger RNA (mRNA) signal recognized by the 30S ribosomal subunit necessary for the initiation of translation?

A. UAA, UAG, and UGA codons
B. Kozak sequence
C. Shine-Dalgarno sequence (Correct Answer)
D. Polyadenosine tail
E. 5' methyl-guanosine cap

Explanation: ***Shine-Dalgarno sequence*** - The **Shine-Dalgarno sequence** is a purine-rich sequence in bacterial mRNA that is complementary to a sequence in the 16S rRNA of the 30S ribosomal subunit. - This interaction correctly positions the **30S ribosomal subunit** on the mRNA, allowing it to find the start codon and initiate translation. *UAA, UAG, and UGA codons* - These are **stop codons** that signal the termination of translation rather than its initiation. - They are recognized by release factors, not the 30S ribosomal subunit, during the elongation phase of protein synthesis. *Kozak sequence* - The **Kozak sequence** is a sequence in eukaryotic mRNA that helps the eukaryotic 40S ribosomal subunit identify the start codon (usually AUG). - It plays an analogous role to the Shine-Dalgarno sequence but functions in **eukaryotic translation initiation**, not prokaryotic. *Polyadenosine tail* - The **poly(A) tail** is a long stretch of adenosine ribonucleotides added to the 3' end of eukaryotic mRNA. - It is involved in mRNA stability, export from the nucleus, and translation efficiency in **eukaryotes**, but it is not a direct signal for ribosomal subunit binding for initiation in prokaryotes. *5' methyl-guanosine cap* - The **5' methyl-guanosine cap** is a modified guanosine nucleotide added to the 5' end of eukaryotic mRNA. - It is crucial for mRNA stability, nuclear export, and ribosome binding to initiate translation in **eukaryotes**, not prokaryotes.

Question 9: A 55-year-old man with type 2 diabetes mellitus comes to the physician because of a 4-day history of fever, chills, nausea, and abdominal pain. He does not use illicit drugs. His temperature is 39°C (102.2°F). Physical examination shows right upper quadrant tenderness. Ultrasonography of the abdomen shows a 6-cm solitary, fluid-filled cavity in the right hepatic lobe. CT-guided percutaneous aspiration of the cavity produces yellowish-green fluid. Culture of the aspirated fluid grows gram-negative, lactose-fermenting rods. Which of the following is the most likely cause of the color of the aspirated fluid?

A. Biliverdin
B. Prodigiosin
C. Staphyloxanthin
D. Pyoverdine
E. Myeloperoxidase (Correct Answer)

Explanation: ***Myeloperoxidase*** - The yellowish-green color of the aspirated fluid, combined with the presence of **gram-negative, lactose-fermenting rods** (suggesting an enteric bacterial infection), indicates a **pyogenic liver abscess**. - **Myeloperoxidase** is an enzyme found in **neutrophils**, which are abundant in pus and contribute to its characteristic yellowish-green hue. *Biliverdin* - **Biliverdin** is a green pigment formed from the breakdown of **heme**, typically associated with jaundice or bile-containing fluids. - While bile might be present if the abscess communicated with the biliary tree, the primary cause of a pus's specific yellowish-green color in a bacterial infection is not biliverdin. *Prodigiosin* - **Prodigiosin** is a distinctive **red pigment** produced by certain bacterial species, most notably *Serratia marcescens*. - This pigment would result in a red or pink discoloration, not the yellowish-green seen in this patient's aspirate. *Staphyloxanthin* - **Staphyloxanthin** is a **golden-yellow pigment** produced by *Staphylococcus aureus*, contributing to its characteristic colony color. - While *S. aureus* can cause abscesses, the culture here grew **gram-negative, lactose-fermenting rods**, making staphyloxanthin an unlikely cause for the color. *Pyoverdine* - **Pyoverdine** is a **fluorescent yellow-green pigment** produced by *Pseudomonas aeruginosa*. - Although it produces a greenish hue, *Pseudomonas aeruginosa* is a **non-lactose-fermenting gram-negative rod**, which contradicts the culture results of lactose-fermenting organisms.

Question 10: An 87-year-old woman presents with fever, fatigue, and blood in her urine. She says that symptoms onset 3 days ago and have not improved. She describes the fatigue as severe and that her urine also has an odd smell to it. She denies any recent history of chills, abdominal or flank pain, or similar past symptoms. Past medical history is significant for a urinary tract infection (UTI) diagnosed 2 weeks ago for which she just completed a course of oral antibiotics. The vitals signs include pulse rate 87/min and temperature 38.8°C (101.8°F). Physical examination is unremarkable. Urinalysis reveals the presence of acid-fast bacilli. The patient is admitted and an appropriate antibiotic regimen is started. Which of the following would be the best test to screen for latent infection by the microorganism most likely responsible for this patient’s condition?

A. Culture in Löwenstein-Jensen media
B. Chest X-ray
C. Interferon-gamma release assays (Correct Answer)
D. Gram stain of urine sample
E. Sputum culture

Explanation: ***Interferon-gamma release assays*** - This assay directly screens for a **T-cell mediated immune response** to *Mycobacterium tuberculosis* antigens, indicating either latent or active infection. - Given the presence of **acid-fast bacilli** in urine and a history suggestive of **renal tuberculosis** (UTI-like symptoms despite antibiotic treatment, fever, fatigue), this test is highly appropriate for detecting latent infection with the causative organism. *Culture in Löwenstein-Jensen media* - This is a culture method used to **isolate and grow mycobacteria** from a sample, which would confirm active infection rather than screen for latent disease. - While it could be used for diagnosis, it's not the best test for *screening for latent infection*. *Chest X-ray* - A chest X-ray is used to detect **pulmonary tuberculosis**, which is the most common form of active TB. - It would not screen for **latent tuberculosis infection** itself or specifically for **extrapulmonary forms** like renal tuberculosis, which this patient likely has. *Gram stain of urine sample* - A Gram stain is effective for identifying common **bacterial pathogens** (Gram-positive or Gram-negative) in urine. - **Mycobacteria** are acid-fast and do **not stain well with Gram stain**, making this test unsuitable for their detection or for screening latent TB. *Sputum culture* - **Sputum culture** is used to diagnose **pulmonary tuberculosis** by detecting *Mycobacterium tuberculosis* in respiratory secretions. - The patient's symptoms are localized to the urinary tract with no respiratory complaints, making sputum culture irrelevant for this presentation.

Question 11: A 44-year-old man presents to urgent care with severe vomiting. He states that he was at a camping ground for a party several hours ago and then suddenly began vomiting profusely. He denies experiencing any diarrhea and otherwise states he feels well. The patient only has a past medical history of lactose intolerance and hypertension managed with exercise and a low salt diet. His temperature is 99.3°F (37.4°C), blood pressure is 123/65 mmHg, pulse is 110/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable only for tachycardia and diffuse abdominal discomfort. Which of the following foods is associated with the most likely cause of this patient's presentation?

A. Egg salad
B. Home-made ice cream
C. Fish
D. Unfiltered stream water
E. Refried rice (Correct Answer)

Explanation: ***Refried rice*** - The rapid onset of symptoms after eating foods like refried rice, especially following social gatherings where food may have been left out, is highly suggestive of **_Bacillus cereus_ food poisoning**. - **_Bacillus cereus_** produces toxins (emetic toxin) that cause **vomiting** within 1-6 hours of ingestion, often without diarrhea. *Egg salad* - Egg salad, particularly if improperly refrigerated, is a common source of **_Salmonella_** infection. - However, **_Salmonella_** typically causes **diarrhea, fever, and abdominal cramps**, often with a longer incubation period of 6-72 hours, which is not consistent with this patient's presentation of rapid onset **vomiting** and no diarrhea. *Home-made ice cream* - Home-made ice cream made with raw eggs can be a source of **_Salmonella_ infection**. - Similar to commercially prepared egg salad, **_Salmonella_** presents with symptoms of **diarrhea, fever**, and a longer incubation period, distinguishing it from this case. *Fish* - Poorly preserved fish can lead to **scombroid poisoning** (histamine toxicity), which can cause rapid onset symptoms including flushing, headache, and gastrointestinal disturbances. - While it can cause rapid symptoms, **scombroid poisoning** typically causes a wider range of symptoms including **rash, flushing, and headaches**, which are absent here. *Unfiltered stream water* - Ingestion of unfiltered stream water increases the risk of parasitic infections such as **_Giardia lamblia_** or bacterial infections like **_E. coli_ (traveler's diarrhea)**. - These infections typically cause **diarrhea, abdominal cramps**, and sometimes nausea and vomiting, but their onset is usually delayed (days to weeks), and diarrhea is a prominent symptom, unlike this patient's presentation.

Question 12: A previously healthy 21-year-old college student is brought to the emergency department because of a 10-hour history of increasing headache, stiff neck, and sensitivity to light. He returned from a mission trip to Haiti 3 weeks ago where he worked in a rural health clinic. He appears lethargic. He is oriented to person, place, and time. His temperature is 39°C (102°F), pulse is 115/min, respirations are 20/min, and blood pressure is 100/70 mm Hg. Examination shows equal and reactive pupils. There are scattered petechiae over the trunk and lower extremities. Range of motion of the neck is decreased due to pain. Neurologic examination shows no focal findings. Blood cultures are obtained and a lumbar puncture is performed. Cerebrospinal fluid (CSF) analysis shows neutrophilic pleocytosis and decreased glucose concentration. Which of the following is most likely to have prevented this patient's condition?

A. Fluconazole therapy
B. Polysaccharide conjugate vaccine (Correct Answer)
C. Inactivated whole-cell vaccine
D. Toxoid vaccine
E. Erythromycin therapy

Explanation: ***Polysaccharide conjugate vaccine*** - The patient presents with classic symptoms of **bacterial meningitis** (**fever, stiff neck, headache, lethargy, petechiae**), and CSF analysis showing **neutrophilic pleocytosis** and **decreased glucose** further supports this diagnosis. - Given the patient's age and travel history, **Neisseria meningitidis** is a strong possibility, and being vaccinated with a **polysaccharide conjugate vaccine** against this bacterium would have likely prevented the condition. *Fluconazole therapy* - **Fluconazole** is an **antifungal medication** used to treat fungal infections, such as candidiasis or cryptococcosis. - It would not be effective against **bacterial meningitis**, which is the most likely diagnosis in this case. *Inactivated whole-cell vaccine* - An **inactivated whole-cell vaccine** is a type of vaccine, but the specific type of pathogen targeted (e.g., *Bordetella pertussis* for whooping cough) is not specified and is unlikely to be relevant here. - While various inactivated vaccines exist, a non-specific inactivated whole-cell vaccine would not necessarily target the organism causing **meningitis** in this patient. *Toxoid vaccine* - A **toxoid vaccine** uses an inactivated bacterial toxin to stimulate an immune response, protecting against diseases caused by bacterial toxins (e.g., tetanus, diphtheria). - This patient's symptoms are indicative of a direct bacterial infection causing **meningitis**, not a toxin-mediated disease. *Erythromycin therapy* - **Erythromycin** is a **macrolide antibiotic** used to treat certain bacterial infections, such as respiratory tract infections (e.g., *Mycoplasma pneumoniae*, *Chlamydia pneumoniae*). - While an antibiotic, it is not the most appropriate first-line treatment or preventative measure for **bacterial meningitis**, especially in the context of preventing **Neisseria meningitidis** infection compared to vaccination.

Question 13: A 31-year-old man with no medical history presents to his provider for infertility. He states that he and his partner have had unprotected intercourse for 1 year and have been unable to conceive. Upon further workup, he is determined to have antisperm antibodies (ASA), but he does not have any other signs or labs suggesting systemic autoimmune disease. A breakdown of which of the following may have played a role in the pathogenesis of his infertility?

A. Connexons
B. Integrins
C. Desmoplakins
D. E-cadherins
E. Occludins (Correct Answer)

Explanation: ***Occludins*** - **Occludins** are key components of **tight junctions** (zona occludens) in the Sertoli cells of the testes, which form the **blood-testis barrier**. - A disruption or breakdown of these tight junctions can expose germ cells to the immune system, leading to the formation of **antisperm antibodies** and subsequent infertility. *Connexons* - **Connexons** form **gap junctions**, which are crucial for intercellular communication through the direct passage of small molecules between adjacent cells. - While important for various cellular processes, their breakdown is not a direct cause of autoantibody formation against sperm. *Integrins* - **Integrins** are cell surface receptors that mediate **cell-matrix** and **cell-cell adhesion**, playing roles in cell signaling and migration. - Their primary function is not in forming the tight junctions that prevent immune exposure of sperm, so their breakdown would not directly cause antisperm antibodies. *Desmoplakins* - **Desmoplakins** are major proteins of **desmosomes**, which provide strong mechanical adhesion between cells through intermediate filaments. - Desmosomes are important for tissue integrity but do not form the selective barrier that protects sperm from immune surveillance. *E-cadherins* - **E-cadherins** are cell adhesion molecules primarily involved in forming **adherens junctions** and maintaining epithelial cell polarity. - While critical for cell-cell adhesion, they are not the primary constituents of the tight junctions that comprise the blood-testis barrier.

Question 14: Immunology researchers attempt to characterize the role of several cytokines in a 5-year-old male’s allergic reaction to peanuts. Months after initial exposure to peanuts, the child was brought to the ER due to repeat exposure with symptoms of anaphylaxis that resolved following epinephrine injection and supportive therapy. Which of the following best describes the role of IL-4 in the child’s response:

A. Neutrophil chemotaxis
B. Macrophage and Th1 cell activation
C. Stimulates IgA production
D. B cell class switching (Correct Answer)
E. Growth of cytotoxic T cells

Explanation: ***B cell class switching*** - **IL-4** is a key cytokine in allergic reactions, primarily promoting **B cell class switching** to IgE, which mediates immediate hypersensitivity. - This **IgE** then binds to mast cells and basophils, leading to histamine release upon re-exposure to the allergen and causing symptoms of anaphylaxis. *Neutrophil chemotaxis* - **Neutrophil chemotaxis** is primarily mediated by cytokines like **IL-8** (CXCL8) and C5a, not IL-4. - Neutrophils are more involved in bacterial infections and acute inflammatory responses, not typically central to allergic anaphylaxis. *Macrophage and Th1 cell activation* - **Macrophage** and **Th1 cell activation** are mainly driven by **interferon-gamma (IFN-γ)** and **IL-12**, which promote cell-mediated immunity. - **IL-4** typically inhibits **Th1 responses** and promotes **Th2 responses**, which are characteristic of allergic reactions. *Stimulates IgA production* - **IgA production** is primarily induced by cytokines such as **transforming growth factor-beta (TGF-β)**, **IL-5**, and **IL-6**, often in mucosal immunity. - While IL-4 plays a role in humoral immunity, its main role in allergic reactions is not the stimulation of IgA. *Growth of cytotoxic T cells* - The **growth of cytotoxic T cells (CTLs)** is primarily stimulated by **IL-2** and **IL-15**, which are crucial for antiviral and antitumor immunity. - **IL-4** is not directly involved in the proliferation or differentiation of cytotoxic T cells.

Question 15: A biology graduate student is performing an experiment in the immunology laboratory. He is researching the recombination activation genes RAG1 and RAG2 in order to verify the function of these genes. He then decides to carry out the experiment on knock-out mice so that these genes will be turned off. Which of the following changes should he be expecting to see?

A. Absence of CD18 in leukocytes
B. Deficiency in CD40L on activated T cells
C. Total lack of B and T cells (Correct Answer)
D. Defect of NADPH oxidase in phagocyte
E. The mice should be asymptomatic

Explanation: ***Total lack of B and T cells*** - **RAG1 and RAG2** genes are essential for **V(D)J recombination**, the process by which B and T cell receptors are diversified. - Without functional RAG genes, **lymphocytes cannot properly mature**, leading to a severe combined immunodeficiency characterized by the absence of B and T cells. *Absence of CD18 in leukocytes* - The absence of **CD18** in leukocytes is characteristic of **Leukocyte Adhesion Deficiency (LAD)**, a disorder affecting neutrophil and lymphocyte adhesion and migration. - This condition is caused by a defect in the **integrin B2 subunit (CD18)**, which is unrelated to RAG genes. *Deficiency in CD40L on activated T cells* - A deficiency in **CD40L (CD154)** on activated T cells is the hallmark of **X-linked Hyper-IgM Syndrome**. - This defect primarily affects B cell class switching and antibody production, with normal numbers of B and T cells initially. *Defect of NADPH oxidase in phagocyte* - A defect in **NADPH oxidase** in phagocytes is responsible for **Chronic Granulomatous Disease (CGD)**. - This leads to impaired phagocytic killing of microorganisms, but does not affect B or T cell development. *The mice should be asymptomatic* - Given the critical role of RAG genes in adaptive immunity, **knock-out mice would suffer from severe immunodeficiency**. - They would be highly susceptible to infections and would likely develop a severe, life-threatening condition, not be asymptomatic.

Question 16: A scientist is studying the process of thymus-dependent B cell activation in humans. He observes that, after bacterial infections, the germinal centers of secondary lymphoid organs become highly metabolically active. After subsequent reinfection with the same pathogen, the organism is able to produce immunoglobulins at a much faster pace. Which of the following processes is likely taking place in the germinal centers at the beginning of an infection?

A. T cell positive selection
B. T cell negative selection
C. Development of early pro-B cells
D. Development of immature B cells
E. Affinity maturation (Correct Answer)

Explanation: ***Affinity maturation*** - This process involves **somatic hypermutation** in the germinal centers, leading to B cells with receptors having higher affinity for the antigen. - Coupled with **clonal selection**, this ensures that subsequent immune responses are faster and more effective due to the improved binding of antibodies to the pathogen. *T cell positive selection* - This process occurs in the **thymic cortex** and selects T cells capable of recognizing self-MHC molecules. - It is crucial for the development of the T cell repertoire and does not occur in germinal centers during B cell activation. *T cell negative selection* - This process takes place primarily in the **thymic medulla** and eliminates T cells that bind too strongly to self-peptide/MHC complexes, preventing autoimmunity. - It is a central tolerance mechanism and is not related to B cell responses in germinal centers. *Development of early pro-B cells* - The development of pro-B cells, and indeed all early stages of B cell development (pro-B, pre-B, immature B), occurs primarily in the **bone marrow**. - These are early developmental stages, distinct from the antigen-driven processes occurring in secondary lymphoid organs during an infection. *Development of immature B cells* - Immature B cells develop from pre-B cells in the **bone marrow** and then migrate to secondary lymphoid organs to complete maturation. - This step occurs prior to encountering an antigen in the germinal centers and is part of initial B cell development rather than the refinement of the immune response to an infection.

Question 17: A 5-year-old boy is referred to an immunologist because of episodes of recurrent infections. He complains of ear pain, nasal discharge, congestion, and headache. His medical history is significant for neonatal sepsis, recurrent bronchitis, and otitis media. The boy also had pneumocystis pneumonia when he was 11 months old. His mother reports that she had a younger brother who had multiple serious infections and died when he was 4 years old because of otogenic sepsis. Her grandfather frequently developed pneumonia and had multiple episodes of diarrhea. The patient is in the 10th percentile for height and 40th percentile for weight. The vital signs include: blood pressure 90/60 mm Hg, heart rate 111/min, respiratory rate 26/min, and temperature 38.3°C (100.9°F). Physical examination reveals a red, swollen, and bulging eardrum and enlarged retroauricular lymph nodes. Meningeal signs are negative and the physician suspects the presence of a primary immunodeficiency. After a thorough laboratory investigation, the patient is found to be CD40L deficient. Despite this deficiency, which of the following chains would still be expressed normally in this patient’s B lymphocytes?

A. α and γ heavy chains
B. μ and δ heavy chains (Correct Answer)
C. α and ε heavy chains
D. μ and ε heavy chains
E. μ heavy chain only

Explanation: ***μ and δ heavy chains*** - **CD40L deficiency** prevents T-cell help for B cells, thus inhibiting **class-switch recombination (CSR)** and the production of IgG, IgA, and IgE. - Despite the inability to class-switch, B cells can still produce **IgM** and **IgD** as these immunoglobulins are generated via alternative splicing of mRNA and do not depend on CD40L signaling or class switching. *α and γ heavy chains* - **Alpha (α) heavy chains** characterize IgA, and **gamma (γ) heavy chains** characterize IgG. - The production of IgA and IgG requires **CD40L-CD40 interaction** for class-switch recombination, which is deficient in this patient. *α and ε heavy chains* - **Alpha (α) heavy chains** characterize IgA, and **epsilon (ε) heavy chains** characterize IgE. - The production of IgA and IgE is dependent on **CD40L-CD40 signaling** for class-switch recombination, which is impaired in CD40L deficiency. *μ and ε heavy chains* - While **mu (μ) heavy chains** are normally expressed, **epsilon (ε) heavy chains** (IgE) require class-switch recombination. - Class switching to IgE requires **CD40L-CD40 interaction** and IL-4, which is defective in this patient. *μ heavy chain only* - While **mu (μ) heavy chains** are indeed expressed, **delta (δ) heavy chains** (IgD) are also normally expressed by naive B cells. - Both IgM and IgD are **co-expressed** on the surface of naive B cells through differential splicing of mRNA, independent of CD40L signaling.

Question 18: A 48-year-old woman presents to an urgent care clinic with the complaints of bloody diarrhea, mid-abdominal discomfort, and cramping for 3 days. She also has nausea, vomiting, and fever. She reports that she had eaten raw oysters at a local seafood restaurant almost 3 days ago, but she denies any other potentially infectious exposures. Her temperature is 37.5°C (99.6°F), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 102/68 mm Hg. Physical examination is non-contributory. What is the most likely diagnosis?

A. Bacillus cereus infection
B. Rotavirus infection
C. Norovirus infection
D. C. difficile colitis
E. Vibrio parahaemolyticus infection (Correct Answer)

Explanation: ***Vibrio parahaemolyticus infection*** - The patient's symptoms of **bloody diarrhea**, **abdominal discomfort**, **nausea**, **vomiting**, and **fever** occurring 3 days after consuming **raw oysters** are highly suggestive of *Vibrio parahaemolyticus* infection. - This bacterium is a common cause of **seafood-associated gastroenteritis**, particularly linked to exposure to **raw shellfish**. *Bacillus cereus infection* - This typically causes **food poisoning** from **reheated rice** or other starchy foods, presenting with a much shorter incubation period (1-6 hours for emetic toxin, 6-15 hours for diarrheal toxin). - While it causes vomiting and diarrhea, the incubation period and association with raw oysters are not consistent with *B. cereus*. *Rotavirus infection* - Rotavirus primarily affects **infants and young children**, causing **watery diarrhea**, vomiting, and fever. - It is highly contagious and spread **fecal-orally**, but it is not typically associated with seafood consumption in adults. *Norovirus infection* - Norovirus is a common cause of **viral gastroenteritis** and can cause outbreaks, often linked to contaminated food or water, but typically not specifically raw oysters. - While it causes nausea, vomiting, and diarrhea, **bloody diarrhea** is uncommon, and the specific exposure to raw oysters points away from norovirus. *C. difficile colitis* - *Clostridium difficile* infection is characterized by **diarrhea** (often foul-smelling and watery) and **abdominal pain**, but it typically occurs after **antibiotic use** or in healthcare settings. - The patient has no history of recent antibiotic use, and the link to raw oysters is inconsistent with *C. difficile*.

Question 19: A 16-year-old man presents to the clinic accompanied by his father, with the complaints of high fever, sore throat, and bloody diarrhea for 4 days. He adds that he is also nauseous and vomited several times in the past 2 days. He denies any recent travel or eating outside. He recently started a dog-walking business. The father relates that two of the dogs had been unwell. His temperature is 38.9°C (102°F), respiratory rate is 16/min, pulse is 77/min, and blood pressure is 100/88 mm Hg. A physical examination is performed and is within normal limits including the abdominal exam. Blood test results are given below: Hb%: 14 gm/dL Total count (WBC): 13,100/mm3 Differential count: Neutrophils: 80% Lymphocytes: 15% Monocytes: 5% What is the most likely diagnosis?

A. Rotavirus infection
B. Norovirus infection
C. C. difficile colitis
D. Bacillus cereus infection
E. Yersiniosis (Correct Answer)

Explanation: ***Yersiniosis*** - The patient's symptoms of **fever**, **sore throat**, **bloody diarrhea**, nausea, and vomiting, combined with a history of exposure to sick dogs (potential carriers of *Yersinia*), strongly suggest **Yersiniosis**. - **Elevated WBC count** with neutrophilia is consistent with a bacterial infection, and *Yersinia enterocolitica* is a known cause of bloody diarrhea, particularly in children and adolescents, often mimicking appendicitis. *Rotavirus infection* - While Rotavirus causes **diarrhea** and vomiting, it is typically **watery diarrhea** and does not usually present with bloody stool or significant neutrophilia. - It is more common in **infants** and young children and symptoms usually resolve within a week. *Norovirus infection* - Norovirus causes acute **gastroenteritis** with vomiting and watery diarrhea, but usually **without blood** in the stool. - The symptoms appear more abruptly and resolve within 1-3 days, and it typically does not present with a significant fever or sore throat. *C. difficile colitis* - This typically occurs after **antibiotic use** and presents with severe **watery diarrhea** (which can occasionally be bloody), abdominal pain, and fever. - The patient has **no history of antibiotic exposure**, making *C. difficile* less likely. *Bacillus cereus infection* - **Bacillus cereus** gastroenteritis is usually associated with **food poisoning**, particularly from reheated rice or other starchy foods. - It presents with either a **vomiting type** (short incubation, severe nausea/vomiting) or a **diarrheal type** (longer incubation, watery diarrhea), neither of which perfectly fit the bloody diarrhea, fever, and sore throat described.

Question 20: A 33-year-old woman presents to the urgent care center with 4 days of abdominal pain and increasingly frequent bloody diarrhea. She states that she is currently having 6 episodes of moderate volume diarrhea per day with streaks of blood mixed in. She says she recently returned from a long camping trip with her friends where they cooked all of their own food and drank water from nearby streams. Physical examination is negative for acute tenderness, rebound tenderness, or abnormal bowel sounds. Her vital signs include temperature 38.0°C (100.4°F), blood pressure 106/74 mm Hg, heart rate 94/min, and respiratory rate 14/min. Given the following options, which is the most likely pathogen responsible for her presentation?

A. Campylobacter
B. Shigella
C. Clostridium difficile
D. Salmonella
E. E. coli O157:H7 (Correct Answer)

Explanation: ***E. coli 0157:H7*** - This patient's presentation with **bloody diarrhea** and a history of consuming potentially contaminated **stream water** during a camping trip is highly suggestive of enterohemorrhagic *E. coli* O157:H7 infection. - While other pathogens can cause bloody diarrhea, the combination of environmental exposure, moderate fever, and the absence of severe systemic illness points towards this specific *E. coli* strain, known for producing toxins that damage the intestinal lining. *Campylobacter* - While *Campylobacter* can cause **bloody diarrhea** and is associated with consuming contaminated water or undercooked poultry, its incubation period is typically longer (2-5 days) than the 4 days described, and it more frequently causes **abdominal cramping with fever**. - Although it's a possibility, *E. coli* O157:H7 is a stronger fit given the rapid onset of bloody diarrhea after likely water exposure. *Shigella* - *Shigella* causes **dysentery** (bloody, mucoid stools) and is highly contagious, often transmitted person-to-person or through contaminated food/water. - However, *Shigella* infections commonly present with **high fever**, severe abdominal cramps, and tenesmus, which are not prominently described in this patient. *Clostridium difficile* - *Clostridium difficile* infection (CDI) is typically associated with **recent antibiotic use** or hospitalization, which is not mentioned in this patient's history. - While CDI can cause bloody diarrhea, it is more commonly characterized by watery diarrhea, severe abdominal pain, and often a distinctive smell. *Salmonella* - *Salmonella* can cause **gastroenteritis** with watery or sometimes bloody diarrhea, often linked to contaminated food (e.g., poultry, eggs). - However, bloody diarrhea is less common than with *E. coli* O157:H7, and this patient's specific exposure to stream water during camping makes *E. coli* O157:H7 a more direct link.

Question 21: A 5-year-old boy presents to the emergency department with a sore throat and trouble breathing. His mother states that his symptoms started last night and have rapidly been worsening. The patient is typically healthy, has received all his childhood immunizations, and currently takes a daily multivitamin. His temperature is 103°F (39.4°C), blood pressure is 100/64 mmHg, pulse is 155/min, respirations are 29/min, and oxygen saturation is 95% on room air. Physical exam is notable for an ill-appearing child who is drooling and is leaning forward to breathe. He does not answer questions and appears very uncomfortable. He will not comply with physical exam to open his mouth for inspection of the oropharynx. Which of the following is the most likely infectious etiology of this patient's symptoms?

A. Candida albicans
B. Haemophilus influenzae (Correct Answer)
C. Streptococcus pneumoniae
D. Epstein-Barr virus
E. Streptococcus viridans

Explanation: ***Haemophilus influenzae*** - This presentation is **classic for acute epiglottitis**: rapid onset of high fever, severe sore throat, **drooling**, **tripod positioning** (leaning forward), and refusal to open mouth due to airway compromise. - *Haemophilus influenzae* type b (Hib) is the **classic and most well-known cause** of epiglottitis, and remains the correct answer for board examination purposes when this clinical presentation is described. - Although Hib vaccination has dramatically reduced the incidence of epiglottitis, cases can still occur due to **vaccine failure**, **waning immunity**, or infection with **non-typeable strains**. - The **"cherry-red epiglottis"** on direct laryngoscopy (when safely performed in the OR) is pathognomonic, though the physical exam should not be forced in an unstable patient. *Candida albicans* - *Candida albicans* causes **oral thrush** (oropharyngeal candidiasis), typically seen in immunocompromised patients or infants. - Presents as **white, removable plaques** on the oral mucosa and tongue, causing discomfort but not acute airway obstruction. - Does not cause the acute, life-threatening presentation of epiglottitis with respiratory distress and drooling. *Streptococcus pneumoniae* - While *Streptococcus pneumoniae* can cause epiglottitis in the post-vaccination era and is increasingly recognized, it is not the **classic board examination answer** for acute epiglottitis. - More commonly causes **pneumonia**, **otitis media**, **sinusitis**, and **meningitis**. - In clinical practice, it may be seen more frequently than Hib in vaccinated populations, but *H. influenzae* remains the prototypical cause tested on examinations. *Epstein-Barr virus* - **Epstein-Barr virus (EBV)** causes **infectious mononucleosis** with fever, exudative pharyngitis, posterior cervical lymphadenopathy, and splenomegaly. - Symptoms develop over days to weeks, not the **rapid progression** (overnight) seen in this case. - While EBV can cause tonsillar hypertrophy, it does not typically produce the acute airway emergency with drooling and tripod positioning characteristic of epiglottitis. *Streptococcus viridans* - **Viridans group streptococci** are normal oral flora and are associated with **dental caries** and **subacute bacterial endocarditis** following dental procedures or in patients with valvular disease. - They are not significant pathogens in acute epiglottitis or upper airway obstruction in previously healthy children.

Question 22: An 8-year-old child is brought to the emergency department because of profuse diarrhea and vomiting that have lasted for 2 days. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. The family recently made a trip to India to visit relatives. Today, his heart rate is 100/min, respiratory rate is 22/min, blood pressure is 105/65 mm Hg, and temperature is 37.2ºC (99.0°F). On physical examination, he appears unwell with poor skin turgor and dry oral mucosa. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. His abdomen is sensitive to shallow and deep palpation. A gross examination of the stool reveals a 'rice water' appearance. Diagnostic microbiology results are pending. Which of the following is the best diagnostic test to aid in the identification of this patient's condition?

A. Methylene blue wet mount
B. Gram stain of stool sample
C. Dark-field microscopy
D. Rapid diagnostic test for cholera toxin
E. Stool culture on TCBS agar (Correct Answer)

Explanation: ***Stool culture on TCBS agar*** - The patient's symptoms (profuse watery diarrhea, vomiting, dehydration, history of travel to India) strongly suggest **cholera**, caused by *Vibrio cholerae*. - **Thiosulfate-citrate-bile salts-sucrose (TCBS) agar** is a highly selective medium specifically used for isolating *Vibrio* species. *Methylene blue wet mount* - This test is primarily used to identify **white blood cells (leukocytes)** in stool, which indicate an inflammatory process, such as in *Shigella* or *Salmonella* infections. - Cholera is a **non-inflammatory** diarrhea, so a methylene blue wet mount would likely be negative for leukocytes and therefore not helpful for diagnosis. *Gram stain of stool sample* - While Gram stain can classify bacteria, it is generally **not useful for diagnosing diarrheal diseases** caused by specific enteric pathogens, as stool contains a vast array of Gram-negative and Gram-positive bacteria. - It would be difficult to identify *Vibrio cholerae* among the normal flora using this method alone. *Dark-field microscopy* - This technique is typically used to visualize **spirochetes**, such as *Treponema pallidum* (syphilis), due to their characteristic motility and morphology. - While *Vibrio cholerae* are motile rods, dark-field microscopy is **not the standard or most sensitive method** for its identification in a stool sample, especially compared to selective cultures. *Rapid diagnostic test for cholera toxin* - While such tests exist and can be useful in epidemic settings for quick screening, they generally have **lower sensitivity and specificity** compared to culture-based methods. - **Culture remains the gold standard** for definitive diagnosis, especially for guiding treatment and epidemiological surveillance.

Question 23: Fifteen years ago, a physician was exposed to Mycobacterium tuberculosis during a medical mission trip to Haiti. A current CT scan of his chest reveals respiratory apical granulomas. The formation of this granuloma helped prevent the spread of the infection to other sites. Which pair of cells contributed to the walling-off of this infection?

A. TH2 cells and macrophages
B. CD8 T cells and NK cells
C. TH1 cells and macrophages (Correct Answer)
D. TH2 cells and neutrophils
E. TH1 cells and neutrophils

Explanation: ***TH1 cells and macrophages*** - **TH1 cells** produce **IFN-γ**, which activates macrophages to become more effective at phagocytosing and killing intracellular pathogens like Mycobacterium tuberculosis. - Activated **macrophages** differentiate into epithelioid cells and multinucleated giant cells, which, along with lymphocytes, form the characteristic **granuloma** to contain the infection. *TH2 cells and macrophages* - **TH2 cells** are primarily involved in humoral immunity and allergic responses, producing cytokines like **IL-4, IL-5, and IL-13**, which are not centrally involved in granuloma formation against intracellular bacteria. - While macrophages are crucial for granuloma formation, their activation in the context of tuberculosis relies on **TH1 cytokine signaling**, rather than TH2. *CD8 T cells and NK cells* - **CD8 T cells** (cytotoxic T lymphocytes) primarily kill infected cells, playing a role in limiting viral infections or intracellular bacteria that escape phagosomes, but are not the main drivers of granuloma formation. - **Natural killer (NK) cells** provide immediate defense against viruses and tumor cells, but are not the primary cellular components responsible for the organized structure of a tuberculous granuloma. *TH2 cells and neutrophils* - **TH2 cells** promote anti-parasitic responses and allergic inflammation, which are not the dominant immune pathways in controlling tuberculosis. - **Neutrophils** are crucial in acute inflammation and bacterial clearance, but they are typically not the primary long-term cellular component of a mature **tuberculous granuloma**; rather, macrophages and T cells predominate. *TH1 cells and neutrophils* - While **TH1 cells** are essential for activating macrophages in tuberculosis, **neutrophils** are not the main effectors of granuloma formation. - **Neutrophils** are more prominent in the early stages of infection and in acute inflammatory responses, but the characteristic structure of a chronic tuberculous granuloma relies on activated macrophages and lymphocytes.

Question 24: A 24-year-old woman presents with 3 days of diarrhea. She was recently on vacation in Peru and admits that on her last day of the trip she enjoyed a dinner of the local food and drink. Upon return to the United States the next day, she developed abdominal cramps and watery diarrhea, occurring about 3-5 times per day. She has not noticed any blood or mucous in her stool. Vital signs are stable. On physical examination, she is well appearing in no acute distress. Which of the following is commonly associated with the likely underlying illness?

A. Raw oysters
B. Soft cheese
C. Unwashed fruits and vegetables (Correct Answer)
D. Fried rice
E. Ground meat

Explanation: ***Unwashed fruits and vegetables*** - The patient's symptoms (watery diarrhea, abdominal cramps, recent travel to Peru, and consuming local food/drink) are highly suggestive of **Traveler's Diarrhea (TD)**. This condition is most commonly caused by **enterotoxigenic E. coli (ETEC)**. - **Unwashed fruits and vegetables** are a common vehicle for the transmission of ETEC and other pathogens associated with TD, as they can be contaminated with fecal matter. *Raw oysters* - **Raw oysters** are typically associated with **Vibrio parahaemolyticus** or **norovirus** infections, which can cause gastroenteritis but are not the most common cause of Traveler's Diarrhea from contaminated food in a country like Peru. - While they can cause diarrhea, the clinical picture is classic for Traveler's Diarrhea, where produce is a more frequent culprit. *Soft cheese* - **Soft cheeses**, especially unpasteurized ones, are more commonly associated with bacterial infections like **Listeria monocytogenes**, which can cause severe illness, but usually presents differently than typical Traveler's Diarrhea, often with fever and systemic symptoms. - They are not a primary source for the common pathogens causing acute watery diarrhea in travelers. *Fried rice* - **Fried rice** is a common source of **Bacillus cereus** food poisoning, which typically causes a very rapid onset of vomiting within 1-6 hours (emetic form) or diarrhea within 6-15 hours (diarrheal form) after consumption. - The patient's symptoms started the day after returning, suggesting a longer incubation period than typically seen with *B. cereus* from fried rice. *Ground meat* - **Ground meat**, particularly undercooked, is a common source of **enterohemorrhagic E. coli (EHEC)**, especially O157:H7, and **Salmonella** or **Campylobacter**. - These typically cause more severe diarrhea, often with **bloody stools**, which the patient explicitly denied.

Question 25: A 3-year-old boy is brought to the physician for the evaluation of recurrent skin lesions. The episodes of lesions started at the age of 3 months. He has also had several episodes of respiratory tract infections, enlarged lymph nodes, and recurrent fevers since birth. The boy attends daycare. The patient's immunizations are up-to-date. He is at the 5th percentile for length and 10th percentile for weight. He appears ill. Temperature is 38°C (100.4°F). Examination shows several raised, erythematous lesions of different sizes over the face, neck, groin, and extremities; some are purulent. Bilateral cervical and axillary lymphadenopathy are present. What is the most likely underlying mechanism of this patient's symptoms?

A. Defective cytoplasmic tyrosine kinase
B. NADPH oxidase deficiency (Correct Answer)
C. Impaired signaling to actin cytoskeleton reorganization
D. Defective neutrophil chemotaxis
E. Impaired repair of double-strand DNA breaks

Explanation: ***NADPH oxidase deficiency*** - The recurrent skin abscesses (purulent lesions), respiratory tract infections, lymphadenopathy, and fevers point to chronic granulomatous disease (CGD), which is caused by a deficiency in **NADPH oxidase**. - **NADPH oxidase** is essential for the production of reactive oxygen species (ROS) in phagocytes, which are critical for killing catalase-positive bacteria and fungi. *Defective cytoplasmic tyrosine kinase* - This mechanism is associated with **X-linked agammaglobulinemia (Bruton's agammaglobulinemia)**, which primarily causes recurrent bacterial infections due to a lack of B cells and antibodies. - While recurrent infections occur, the typical presentation involves encapsulated bacteria and lacks the widespread purulent skin lesions and lymphadenopathy seen in CGD. *Impaired signaling to actin cytoskeleton reorganization* - This defect is characteristic of **Wiskott-Aldrich syndrome**, leading to thrombocytopenia, eczema, and recurrent infections, particularly by encapsulated bacteria. - The clinical picture of recurrent widespread skin abscesses and granuloma formation is not typical for Wiskott-Aldrich syndrome. *Defective neutrophil chemotaxis* - This can be seen in conditions like **leukocyte adhesion deficiency (LAD)** or **Chédiak-Higashi syndrome**. - LAD presents with recurrent bacterial infections, impaired wound healing, and delayed umbilical cord separation, while Chédiak-Higashi involves partial oculocutaneous albinism and recurrent pyogenic infections, distinct from this patient's symptoms. *Impaired repair of double-strand DNA breaks* - This defect is associated with conditions like **ataxia-telangiectasia**, which involves cerebellar ataxia, telangiectasias, and immunodeficiency (T-cell and IgA deficiency). - The patient's symptoms of recurrent purulent skin lesions and infections are not characteristic of the DNA repair defects seen in ataxia-telangiectasia.

Question 26: A 23-year-old woman goes to a walk-in clinic while on the fourth day of her honeymoon. She is very upset saying that her honeymoon is being ruined because she is in severe pain. She states that yesterday she began to experience severe pain with urination and seems to be urinating more frequently than normal. She does admit that she has been having increased sexual intercourse with her new husband while on their honeymoon. The physician diagnoses the patient and prescribes trimethoprim-sulfamethoxazole. Which of the following virulence factors is most likely responsible for this patient's infection?

A. Exotoxin
B. Flagella
C. K capsule
D. P fimbriae (Correct Answer)
E. LPS endotoxin

Explanation: ***P fimbriae*** - The patient presents with classic **acute cystitis** (UTI), commonly caused by **uropathogenic E. coli (UPEC)**, especially in sexually active women ("honeymoon cystitis"). - **Fimbrial adhesins are the most critical virulence factor** for E. coli UTIs, enabling bacteria to adhere to uroepithelial cells and resist being flushed out by urine flow. - Note: **Type 1 fimbriae** (which bind mannose on bladder epithelium) are actually the primary adhesins for **cystitis**, while **P fimbriae** (which bind P blood group antigens) are more specific for **pyelonephritis** (upper UTI). - In this question context, "P fimbriae" represents the broader concept of **fimbrial adhesion** as the key mechanism—making it the best answer among the options provided, as adhesion is essential for colonization and infection. *Exotoxin* - E. coli causing uncomplicated UTIs do not rely on exotoxin production as their primary pathogenic mechanism. - Exotoxins are associated with diseases like diphtheria, botulism, or cholera with different clinical presentations. *Flagella* - **Flagella** provide motility, helping bacteria ascend the urinary tract. - However, motility is secondary to **adhesion**—bacteria must first attach to uroepithelium to establish infection; without fimbriae, they would be washed away regardless of motility. *K capsule* - The **K capsule** (polysaccharide capsule) provides antiphagocytic properties and serum resistance, particularly important in invasive infections like **pyelonephritis** or **bacteremia**. - While it contributes to virulence in severe UTIs, **adhesion is the critical first step** for establishing bladder colonization in cystitis. *LPS endotoxin* - **LPS (lipopolysaccharide)** is a structural component of Gram-negative bacteria that triggers inflammatory responses when released. - LPS causes systemic symptoms (fever, shock) in severe infections or sepsis, but it is not the primary virulence factor responsible for the **localized bladder symptoms** (dysuria, frequency) characteristic of simple cystitis. - The infection establishes via adhesion first; LPS effects are secondary inflammatory consequences.

Question 27: A 52-year-old male presents with recent weight loss, fever, and joint pain. He reports frequent bouts of diarrhea. An intestinal biopsy demonstrates PAS-positive, non-acid fast macrophage inclusions in the lamina propria. Which of the following organisms is likely responsible for this patient’s illness:

A. Giardia lamblia
B. Campylobacter jejuni
C. Tropheryma whippleii (Correct Answer)
D. Ascaris lumbricoides
E. Mycobacterium avium-intracellulare complex

Explanation: ***Tropheryma whippleii*** - The constellation of **weight loss**, **fever**, **joint pain**, **diarrhea**, and specifically a biopsy showing **PAS-positive, non-acid-fast macrophage inclusions** in the lamina propria is pathognomonic for **Whipple's disease**, caused by *Tropheryma whippleii*. - These macrophages are engorged with undigested bacterial components, which stain magenta with **Periodic acid–Schiff (PAS)** stain due to their bacterial cell wall glycolipids. *Giardia lamblia* - This parasite commonly causes **diarrhea** and malabsorption but does not typically lead to systemic symptoms like fever and arthritis. - Intestinal biopsies would show **trophozoites** or **cysts** in the lumen or attached to the mucosa, not PAS-positive macrophage inclusions. *Campylobacter jejuni* - A common cause of **bacterial gastroenteritis**, presenting with bloody diarrhea, fever, and abdominal pain. - While it can cause **reactive arthritis**, its presence in the intestine does not lead to **PAS-positive macrophage inclusions** in the lamina propria; rather, it causes acute inflammatory changes. *Ascaris lumbricoides* - This is a large intestinal nematode that can cause abdominal pain, malnutrition, and bowel obstruction in heavy infections. - It does not cause systemic symptoms like **fever** and **arthritis**, nor does it result in distinct macrophage inclusions on biopsy. *Mycobacterium avium-intracellulare complex* - While **MAC** can cause intestinal infection, particularly in immunocompromised individuals, leading to malabsorption and diarrhea, its characteristic feature on biopsy would be **acid-fast positive** macrophages. - The question specifically states **non-acid-fast** inclusions, differentiating it from MAC infection.

Question 28: A 43-year-old man with a history of chronic alcoholism presents with a chronic cough and dyspnea. He says he traveled to Asia about 4 months ago and his symptoms started shortly after he returned. His temperature is 40.2°C (104.4°F) and pulse is 92/min. Physical examination reveals poor personal hygiene and a cough productive of foul blood-streaked sputum. Auscultation reveals decreased breath sounds on the right. A chest radiograph reveals an ill-defined circular lesion in the right middle lobe. Which of the following is true regarding this patient’s most likely diagnosis?

A. A positive tuberculin test would be diagnostic of active infection.
B. DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.
C. Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks. (Correct Answer)
D. Stains of gastric washing and urine have a high diagnostic yield on microscopy.
E. Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.

Explanation: ***Inoculation of a sputum sample into selective agar media needs to be incubated at 35–37°C (95.0–98.6°F) for up to 8 weeks.*** - The patient's symptoms, travel history, imaging findings (ill-defined circular lesion), and social history (chronic alcoholism, poor hygiene) strongly suggest **pulmonary tuberculosis (TB)** with potential cavitation. - **Mycobacterium tuberculosis** is a slow-growing organism; therefore, **culture on selective media** (e.g., Lowenstein-Jensen or Middlebrook media) for an extended period (typically 6-8 weeks) is the gold standard for diagnosis, allowing for identification and drug susceptibility testing. *A positive tuberculin test would be diagnostic of active infection.* - A **positive tuberculin skin test (TST)** or **interferon-gamma release assay (IGRA)** indicates **TB exposure** or **latent TB infection**, not necessarily active disease. - Many individuals with latent TB will have a positive test but no active infection, requiring further diagnostic workup like sputum microscopy and culture to confirm active disease. *DNA polymerase chain reaction (PCR) has poor sensitivity when applied to smear positive specimens.* - **DNA PCR** is a rapid and highly **sensitive** and **specific** test for M. tuberculosis, especially valuable for **smear-positive specimens**. - It can detect very small amounts of mycobacterial DNA, significantly reducing the diagnostic time compared to culture. *Stains of gastric washing and urine have a high diagnostic yield on microscopy.* - While M. tuberculosis can be found in gastric washings, especially in patients who cannot produce sputum, its **diagnostic yield for pulmonary TB by microscopy is not high** compared to sputum. - **Urine microscopy** is rarely used for the diagnosis of pulmonary tuberculosis; it is only relevant in cases of **renal or genitourinary tuberculosis**. *Ziehl-Neelsen staining is more sensitive than fluorescence microscopy with auramine-rhodamine stain.* - **Fluorescence microscopy with auramine-rhodamine stain** is typically **more sensitive** than Ziehl-Neelsen staining for detecting acid-fast bacilli (AFB). - The fluorescent stain allows for lower magnification scanning and better visualization of the bacilli, leading to a higher detection rate.

Question 29: An 87-year-old woman is brought to the emergency department from her nursing home because of increasing confusion and lethargy for 12 hours. The nursing home aide says she did not want to get out of bed this morning and seemed less responsive than usual. She has Alzheimer's disease, hypertension, and a history of nephrolithiasis. She has chronic, intractable urinary incontinence, for which she has an indwelling urinary catheter. Current medications include galantamine, memantine, and ramipril. Her temperature is 38.5°C (101.3°F), pulse is 112/min, respiratory rate is 16/min, and blood pressure is 108/76 mm Hg. Physical examination shows mild tenderness to palpation of the lower abdomen. On mental status examination, she is oriented only to person. Laboratory studies show: Hemoglobin 12.4 g/dL Leukocyte count 9,000/mm3 Platelet count 355,000/mm3 Urine pH 8.2 Glucose 1+ Protein 2+ Ketones negative RBC 5/hpf WBC 35/hpf Bacteria moderate Nitrites positive Which of the following is the most likely causal organism?

A. Enterococcus faecalis
B. Klebsiella pneumoniae
C. Staphylococcus saprophyticus
D. Escherichia coli
E. Proteus mirabilis (Correct Answer)

Explanation: ***Proteus mirabilis*** - The high urine pH (8.2), positive nitrites, and moderate bacteria, along with signs of infection in an elderly catheterized patient, are highly suggestive of a **urea-splitting organism**. - **Proteus mirabilis** is a common cause of catheter-associated UTIs and produces urease, leading to alkaline urine and the formation of struvite stones, consistent with the patient's history of nephrolithiasis. *Enterococcus faecalis* - While *Enterococcus faecalis* can cause UTIs, it typically does not produce urease and therefore would not cause such a **markedly elevated urine pH** (above 7.5). - Although it can cause positive nitrites, the absence of a strong alkali pH makes it less likely than *Proteus mirabilis*. *Klebsiella pneumoniae* - *Klebsiella pneumoniae* can cause UTIs and produce nitrites, but it is not typically a strong **urease producer** to the extent that would cause an alkaline urine pH of 8.2. - It is more commonly associated with nosocomial infections, but the highly alkaline urine points away from it as the most likely cause here. *Staphylococcus saprophyticus* - *Staphylococcus saprophyticus* is a common cause of UTIs in young, sexually active women, but it is **rare in elderly, catheterized patients**. - It is also not typically associated with such a high urine pH as seen in this case. *Escherichia coli* - *Escherichia coli* is the most common cause of UTIs, but it is a **non-urease-producing** bacterium and would typically result in acidic urine, or at least a less alkaline pH than 8.2. - While it would cause positive nitrites and moderate bacteria, the elevated pH makes it less likely than *Proteus mirabilis* in this context.

Question 30: A 22-year-old man comes to the physician because of an ulcer on his penis for 12 days. The ulcer is painful and draining yellow purulent material. He returned from a study abroad trip to India 3 months ago. His immunizations are up-to-date. He is sexually active with one female partner and uses condoms inconsistently. He appears uncomfortable. His temperature is 37.2°C (99.0°F), pulse is 94/min, and blood pressure is 120/80 mm Hg. Examination shows tender inguinal lymphadenopathy. There is a 2-cm ulcer with a necrotic base proximal to the glans of the penis. Which of the following is the most likely causal organism?

A. Klebsiella granulomatis
B. Haemophilus ducreyi (Correct Answer)
C. Herpes simplex virus 2
D. Treponema pallidum
E. Chlamydia trachomatis

Explanation: ***Haemophilus ducreyi*** - The presentation of a **single, painful penile ulcer** with a **necrotic base** and **tender inguinal lymphadenopathy** is classic for **chancroid**, - This condition is caused by **Haemophilus ducreyi**, and the patient's recent travel to India, where chancroid is endemic, increases the likelihood of this diagnosis. *Klebsiella granulomatis* - This bacterium causes **granuloma inguinale (donovanosis)**, which typically presents with **painless, beefy-red ulcers** that bleed easily, - The ulcer described in the patient is **painful** and has a **necrotic base**, which is inconsistent with donovanosis. *Herpes simplex virus 2* - **Herpes simplex virus (HSV-2)** typically causes **multiple, painful vesicular lesions** that rupture to form shallow ulcers, often accompanied by systemic symptoms like fever and malaise. - The patient describes a **single, large ulcer** with a necrotic base, which is not characteristic of herpetic lesions. *Treponema pallidum* - **Treponema pallidum** causes **syphilis**, which presents as a **painless chancre** with a clean base and firm, non-tender lymphadenopathy in its primary stage. - The patient's ulcer is explicitly described as **painful** and draining **purulent material**, ruling out a syphilitic chancre. *Chlamydia trachomatis* - Certain serovars of **Chlamydia trachomatis** cause **lymphogranuloma venereum (LGV)**, which initially presents as a transient, **painless papule or ulcer** that often goes unnoticed, followed by significant, painful inguinal lymphadenopathy (buboes). - While LGV involves painful lymphadenopathy, the initial ulcer is typically small and unnoticed, and the described ulcer is large, painful, and has a necrotic base, which is not characteristic of LGV.

Question 31: A 62-year-old man comes to the physician because of a 2-day history of fever, chills, and flank pain. Five days ago, he was catheterized for acute urinary retention. His temperature is 39.3°C (102.7°F). Physical examination shows right-sided costovertebral angle tenderness. Urine studies show numerous bacteria and WBC casts. Urine culture on blood agar grows mucoid, gray-white colonies. Urine culture on eosin methylene blue agar grows purple colonies with no metallic green sheen. Which of the following is the most likely causal pathogen?

A. Escherichia coli
B. Klebsiella pneumoniae (Correct Answer)
C. Pseudomonas aeruginosa
D. Proteus mirabilis
E. Staphylococcus saprophyticus

Explanation: ***Klebsiella pneumoniae*** - The presence of **mucoid, gray-white colonies** on blood agar and **purple colonies with no metallic green sheen** on EMB agar, along with a history of catheterization, fever, and flank pain strongly suggests *Klebsiella pneumoniae*. - *Klebsiella* is a common cause of **catheter-associated UTIs** and often produces mucoid colonies due to its capsule. *Escherichia coli* - *E. coli* typically produces **metallic green sheen** on EMB agar due to rapid lactose fermentation, which is absent in this case. - While *E. coli* is a common cause of UTIs, the specific culture findings differentiate it from *Klebsiella*. *Pseudomonas aeruginosa* - *Pseudomonas* often produces a **grape-like odor** and distinctive **blue-green pigment** on agar, neither of which is mentioned. - It does not ferment lactose and would thus not produce purple colonies on EMB, but rather appear as colorless or clear colonies. *Proteus mirabilis* - *Proteus mirabilis* is known for its **swarming motility** on agar, which creates a characteristic spreading growth pattern, not merely mucoid colonies. - It also produces **urease**, which can lead to alkaline urine and struvite stones, but the distinguishing colony morphology is not met. *Staphylococcus saprophyticus* - *Staphylococcus saprophyticus* is a **Gram-positive coccus** and would not grow purple colonies on EMB agar, which is selective for Gram-negative bacteria. - It is a common cause of UTIs in young, sexually active women, which does not fit the patient's demographic.

Question 32: An otherwise healthy 7-year-old boy is brought to the emergency department because of a 1-day history of involuntary muscle contractions and pain in his back and neck. Two weeks ago, he fell while playing in the sandbox and scraped both his knees. He has not received any vaccinations since birth. His temperature is 38.5°C (101.3°F). He is diaphoretic. Examination shows inability to open his mouth beyond 1 cm. There is hyperextension of the lumbar spine and resistance to neck flexion. Administration of which of the following would most likely have prevented this patient's current condition?

A. Chemically-inactivated virus
B. Denatured bacterial product (Correct Answer)
C. Viable but weakened microorganism
D. Human immunoglobulin against a viral protein
E. Capsular polysaccharides

Explanation: ***Denatured bacterial product*** - This describes a **toxoid vaccine**, specifically the **tetanus toxoid vaccine**, which is a denatured form of the bacterial toxin. - The patient's symptoms (trismus, back and neck pain, muscle contractions, **opisthotonus**) are classic for **tetanus**, and his unvaccinated status and recent wound increase his risk. *Chemically-inactivated virus* - This refers to an **inactivated viral vaccine**, which is effective against viral infections, not bacterial toxins. - Tetanus is caused by a bacterial toxin, not a virus, making this vaccine type irrelevant to preventing the described condition. *Viable but weakened microorganism* - This describes a **live-attenuated vaccine**, which typically induces a strong immune response against the live pathogen itself. - Tetanus prevention targets the toxin produced by *Clostridium tetani*, not the microorganism directly via an attenuated form. *Human immunoglobulin against a viral protein* - This describes **passive immunization** using antibodies against a viral protein, usually for viral infections or post-exposure prophylaxis. - While passive immunization with tetanus immune globulin can *treat* tetanus, a vaccine is needed for *prevention*, and the target here is a bacterial toxin, not a viral protein. *Capsular polysaccharides* - This describes a component of **polysaccharide vaccines** used against encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae type b*). - *Clostridium tetani* is not an encapsulated bacterium, and its pathogenicity stems from its toxin, not its capsule.

Question 33: A 13-year-old boy is brought to the emergency department because of vomiting, diarrhea, abdominal pain, and dizziness for the past 3 hours with fever, chills, and muscle pain for the last day. He had presented 5 days ago for an episode of epistaxis caused by nasal picking and was treated with placement of anterior nasal packing. His parents report that the bleeding stopped, but they forgot to remove the nasal pack. His temperature is 40.0°C (104.0°F), pulse is 124/min, respirations are 28/min, and blood pressure is 96/68 mm Hg. He looks confused, and physical exam shows conjunctival and oropharyngeal hyperemia with a diffuse, erythematous, macular rash over the body that involves the palms and the soles. Removal of the anterior nasal pack shows hyperemia with purulent discharge from the underlying mucosa. Laboratory studies show: Total white blood cell count 30,000/mm3 (30 x 109/L) Differential count Neutrophils 90% Lymphocytes 8% Monocytes 1% Eosinophils 1% Basophils 0% Platelet count 95,000/mm3 (95 x 109/L) Serum creatine phosphokinase 400 IU/L What is the most likely diagnosis for this patient?

A. Stevens-Johnson syndrome
B. Measles
C. Disseminated gonococcal infection
D. Herpes simplex virus type 2 (HSV-2) meningitis
E. Toxic shock syndrome (Correct Answer)

Explanation: ***Toxic shock syndrome*** - The patient's presentation with **fever**, **hypotension**, **diffuse erythematous rash** involving palms/soles, **multisystem involvement** (vomiting, diarrhea, dizziness, confusion, elevated CPK), and the history of prolonged **nasal packing** (a common nidus for *Staphylococcus aureus* toxin production) is highly characteristic of **toxic shock syndrome (TSS)**. - **Leukocytosis with neutrophilia** and **thrombocytopenia** are also common laboratory findings in TSS. *Stevens-Johnson syndrome* - Characterized by **mucocutaneous lesions** with epidermal detachment, forming **bullae** and **erosions**, often preceded by fever and flu-like symptoms. - While it can involve mucous membranes, the **diffuse erythematous macular rash without bullae** and the rapid development of **shock** are not typical features. *Measles* - Presents with a **maculopapular rash** that typically starts on the face and spreads downwards, often coalescing. It is preceded by **prodromal symptoms** like cough, coryza, conjunctivitis, and **Koplik spots**. - **Hypotension**, **severe multiorgan dysfunction**, and **nasal packing as a risk factor** are not features of measles. *Disseminated gonococcal infection* - Can cause **fever**, migratory **polyarthralgia**, and a **pustular or vesiculopustular rash** with hemorrhagic lesions, primarily on the extremities. - The described **diffuse erythematous macular rash**, severe hypotension, and history of nasal packing do not fit the typical presentation of disseminated gonococcal infection. *Herpes simplex virus type 2 (HSV-2) meningitis* - Primarily causes **aseptic meningitis** with symptoms like fever, headache, stiff neck, and photophobia. - It does not explain the **diffuse erythematous rash**, **hypotension**, **multisystem involvement**, or the role of **nasal packing** in the patient's presentation.

Question 34: Antigen presentation of extracellular pathogens by antigen presenting cells requires endocytosis of the antigen, followed by the degradation in the acidic environment of the formed phagolysosome. Should the phagolysosome become unable to lower its pH, what is the most likely consequence?

A. Deficient presentation of pathogens to CD4 T-cells (Correct Answer)
B. Deficient cell extravasation
C. Deficient presentation of pathogens to CD8 T-cells
D. Deficient NK cell activation
E. Deficient expression of B7

Explanation: ***Deficient presentation of pathogens to CD4 T-cells*** - The acidic environment of the **phagolysosome** is crucial for optimal **antigen degradation** and processing into peptides that can bind to **MHC class II molecules**. - Without proper acidification, peptide loading onto **MHC class II** is impaired, leading to deficient presentation of extracellular pathogens to **CD4 T-cells**. *Deficient cell extravasation* - **Cell extravasation** involves events like rolling, adhesion, and transendothelial migration, which are primarily regulated by **adhesion molecules** and **chemokines**, not phagolysosomal pH. - A defect in phagolysosomal pH would not directly impede the ability of cells to exit the vasculature. *Deficient presentation of pathogens to CD8 T-cells* - **CD8 T-cell** activation primarily involves the presentation of **intracellular antigens** via **MHC class I molecules**, which typically occurs through degradation in the **cytosol** via proteasomes. - While some cross-presentation pathways exist, the primary mechanism of CD8 T-cell antigen presentation is not dependent on the acidification of phagolysosomes for extracellular pathogens. *Deficient NK cell activation* - **Natural Killer (NK) cells** recognize and kill target cells based on the presence or absence of **MHC class I molecules** and activating ligands, not on the processing of extracellular antigens within phagolysosomes. - Their activation depends on cytokine environments and surface receptor interactions, not directly on phagolysosomal pH. *Deficient expression of B7* - **B7 molecules (CD80/CD86)** are **co-stimulatory molecules** expressed by antigen-presenting cells that are crucial for full T-cell activation. While antigen processing can influence APC activation, a specific defect in phagolysosomal pH would primarily affect the *presentation* of peptides, not the *expression* of co-stimulatory molecules. - The expression of B7 is more broadly regulated by inflammatory signals and toll-like receptor (TLR) engagement, rather than being solely dependent on proper phagolysosomal acidification.

Question 35: A 15-year-old boy presents to the clinic complaining of an uncomfortable skin condition that started 2 years ago. The patient states that his skin feels ‘oily’ and that he is embarrassed by his appearance. On examination, he is a healthy-looking teenager who has reached the expected Tanner stage for his age. The skin on his face and back is erythematous and shows signs of inflammation. What is the microbiologic agent most associated with this presentation?

A. Human papillomavirus (HPV) strains 2 and 4
B. Cutibacterium acnes (Correct Answer)
C. Bartonella henselae
D. HHV-8
E. Streptococcus pyogenes

Explanation: ***Cutibacterium acnes*** - The presentation of "oily" skin, erythematous, inflamed skin on the face and back in a 15-year-old boy is highly suggestive of **acne vulgaris**. - **Cutibacterium acnes** (formerly *Propionibacterium acnes*) is a commensal bacterium that thrives in the anaerobic, lipid-rich environment of the pilosebaceous unit, and its proliferation contributes significantly to the inflammatory response seen in acne. *Human papillomavirus (HPV) strains 2 and 4* - HPV strains 2 and 4 are typically associated with **cutaneous warts**, which manifest as benign skin growths and are not characterized by widespread oily, erythematous, and inflamed skin. - The clinical picture of acne, characterized by **comedones, papules, pustules, and sometimes cysts**, is distinct from the lesions caused by HPV. *Bartonella henselae* - This bacterium is the causative agent of **cat-scratch disease**, which typically presents with localized lymphadenopathy following a cat scratch or bite. - While it can manifest with unusual skin lesions (e.g., bacillary angiomatosis in immunocompromised individuals), it does not cause generalized oily, inflammatory acne. *HHV-8* - Human Herpesvirus 8 (HHV-8) is primarily associated with **Kaposi's sarcoma**, a vascular malignancy characterized by reddish-purple skin lesions, most commonly in immunocompromised individuals. - It does not cause acne vulgaris or the described skin changes. *Streptococcus pyogenes* - *Streptococcus pyogenes* is known for causing a variety of skin infections such as **impetigo, cellulitis, and erysipelas**, which are acute bacterial infections. - These infections typically present with rapidly developing, painful, and often pustular or vesicular lesions, distinct from the chronic, oily, inflammatory papules and pustules of acne.

Question 36: A 5-year-old girl is brought to the physician because of watery discharge from her right eye for 2 weeks. She and her parents, who are refugees from Sudan, arrived in Texas a month ago. Her immunization status is not known. She is at the 25th percentile for weight and the 50th percentile for height. Her temperature is 37.2°C (99°F), pulse is 90/min, and respirations are 18/min. Examination of the right eye shows matting of the eyelashes. Everting the right eyelid shows hyperemia, follicles, and papillae on the upper tarsal conjunctiva. Slit-lamp examination of the right eye shows follicles in the limbic region and the bulbar conjunctiva. There is corneal haziness with neovascularization at the 12 o'clock position. Examination of the left eye is unremarkable. Direct ophthalmoscopy of both eyes shows no abnormalities. Right pre-auricular lymphadenopathy is present. Which of the following is the most likely diagnosis in this patient?

A. Neisserial conjunctivitis
B. Trachoma conjunctivitis (Correct Answer)
C. Acute herpetic conjunctivitis
D. Angular conjunctivitis
E. Acute hemorrhagic conjunctivitis

Explanation: ***Trachoma conjunctivitis*** - The constellation of **follicles and papillae on the upper tarsal conjunctiva**, **limbal follicles**, **corneal haziness with neovascularization (pannus)**, and **pre-auricular lymphadenopathy** in a child from an endemic region (Sudan) is classic for **trachoma**. - This chronic form of conjunctivitis is caused by *Chlamydia trachomatis* serovars A, B, and C, leading to progressive scarring that can eventually cause **trichiasis** and blindness. *Neisserial conjunctivitis* - This condition typically presents with **hyperacute onset**, **copious purulent discharge**, and significant eyelid swelling, often within days of birth or infection. - While it can cause corneal involvement, the chronic follicular and papillary changes with limbal follicles and pannus are not characteristic. *Acute herpetic conjunctivitis* - Usually presents with **unilateral follicular conjunctivitis**, often accompanied by **periorbital vesicles** or a history of cold sores. - While it can cause corneal involvement (typically **dendritic ulcers**), the specific follicular changes, presence of papillae, and chronic course leading to pannus seen here are not typical. *Angular conjunctivitis* - Characterized by **redness, excoriation, and maceration** primarily localized to the **outer canthus** (angle) of the eye, often caused by *Moraxella lacunata* or *Staphylococcus aureus*. - It does not present with the diffuse follicular and papillary changes, limbal follicles, or corneal neovascularization described in this patient. *Acute hemorrhagic conjunctivitis* - This is typically an **acute, highly contagious viral conjunctivitis** characterized by **subconjunctival hemorrhages**, rapid onset, and usually resolves spontaneously. - It does not cause chronic follicular changes, limbal follicles, or corneal neovascularization, and the duration in this patient (2 weeks) suggests a more chronic process.

Question 37: An investigator is studying bacterial toxins in a nonpathogenic bacterial monoculture that has been inoculated with specific bacteriophages. These phages were previously cultured in a toxin-producing bacterial culture. After inoculation, a new toxin is isolated from the culture. Genetic sequencing shows that the bacteria have incorporated viral genetic information, including the gene for this toxin, into their genome. The described process is most likely responsible for acquired pathogenicity in which of the following bacteria?

A. Staphylococcus aureus
B. Haemophilus influenzae
C. Neisseria meningitidis
D. Streptococcus pneumoniae
E. Corynebacterium diphtheriae (Correct Answer)

Explanation: ***Corynebacterium diphtheriae*** - The process described, where a bacterium acquires new genetic information (e.g., a toxin gene) from a bacteriophage, is called **lysogenic conversion** or **phage conversion**. *Corynebacterium diphtheriae* is the **classic example** of this mechanism, acquiring its toxigenicity through phage-mediated transfer of the **diphtheria toxin gene (tox gene)** via bacteriophage β. - The diphtheria toxin is an **AB toxin** that ADP-ribosylates and thereby inactivates **elongation factor 2 (EF-2)**, inhibiting host cell protein synthesis and leading to the characteristic symptoms of diphtheria. - This is the **prototypical and most clinically significant example** of lysogenic conversion in medical microbiology. *Staphylococcus aureus* - While *Staphylococcus aureus* can acquire some virulence factors via bacteriophages (e.g., **Panton-Valentine leukocidin**, some enterotoxins), many of its toxins are encoded on **mobile genetic elements** such as plasmids, pathogenicity islands, or chromosomal genes. - However, *S. aureus* is **not the classic example** of lysogenic conversion described in this scenario. *C. diphtheriae* better exemplifies the acquisition of a major toxin exclusively through phage conversion. *Haemophilus influenzae* - *Haemophilus influenzae* primarily causes disease through its **polysaccharide capsule** (especially type b) and is a common cause of respiratory infections and meningitis. - Its major virulence factors are typically chromosomally encoded or acquired through **transformation** (uptake of naked DNA), not through phage conversion for a primary toxin. *Neisseria meningitidis* - *Neisseria meningitidis* causes meningococcal disease, primarily due to its **polysaccharide capsule** and **endotoxin (LPS)**. - While genetic exchange can occur, the acquisition of a major toxin gene by phage conversion as described is not a primary mechanism for its key virulence factors. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a leading cause of pneumonia, meningitis, and otitis media, with its main virulence factor being its **polysaccharide capsule**. - It primarily acquires genetic material through **transformation** (competence-mediated uptake of naked DNA), which contributes to antibiotic resistance and capsule types, but lysogenic conversion with toxin acquisition is not typical for its major virulence factors.

Question 38: Which of the following patient presentations would be expected in an infant with defective LFA-1 integrin (CD18) protein on phagocytes, in addition to recurrent bacterial infections?

A. Cardiac defects, hypoparathyroidism, palatal defects, and learning disabilities
B. Chronic diarrhea, oral candidiasis, severe infections since birth, absent thymic shadow
C. Progressive neurological impairment and cutaneous telangiectasia
D. Skin infections with absent pus formation, delayed umbilicus separation (Correct Answer)
E. Eczema and thrombocytopenia

Explanation: ***Skin infections with absent pus formation, delayed umbilicus separation*** - A defect in **LFA-1 integrin (CD18)** prevents phagocytes from adhering to endothelial cells and migrating to sites of infection, leading to **absent pus formation** despite severe infections. - **Delayed umbilical cord separation** (typically >30 days) is a classic sign due to impaired neutrophil recruitment at the site of cord detachment. *Cardiac defects, hypoparathyroidism, palatal defects, and learning disabilities* - This constellation of symptoms is characteristic of **DiGeorge syndrome**, which involves a defect in T-cell development due to thymic aplasia/hypoplasia. - These specific defects are not directly caused by LFA-1 integrin deficiency. *Chronic diarrhea, oral candidiasis, severe infections since birth, absent thymic shadow* - These symptoms are highly suggestive of **Severe Combined Immunodeficiency (SCID)**, which involves a profound defect in both B and T cell immunity. - SCID presents with a broader spectrum of opportunistic infections and developmental issues not directly related to integrin function. *Progressive neurological impairment and cutaneous telangiectasia* - These are hallmark features of **Ataxia-Telangiectasia**, a genetic disorder affecting DNA repair and leading to immune deficiencies and cerebellar degeneration. - This condition primarily involves T-cell dysfunction and increased cancer risk, not LFA-1 integrin deficiency. *Eczema and thrombocytopenia* - The combination of **eczema** (dermatitis) and **thrombocytopenia** (low platelet count), along with recurrent infections, is characteristic of **Wiskott-Aldrich syndrome**. - This syndrome is caused by a defect in the WASP protein, affecting immune cell function and platelet formation, distinct from LFA-1 integrin deficiency.

Question 39: A 6-year-old girl is brought to the physician for intermittent fevers and painful swelling of the left ankle for 2 weeks. She has no history of trauma to the ankle. She has a history of sickle cell disease. Current medications include hydroxyurea and acetaminophen for pain. Her temperature is 38.4°C (101.2°F) and pulse is 112/min. Examination shows a tender, swollen, and erythematous left ankle with point tenderness over the medial malleolus. A bone biopsy culture confirms the diagnosis. Which of the following is the most likely causal organism?

A. Coccidioides immitis
B. Pseudomonas aeruginosa
C. Escherichia coli
D. Streptococcus pneumoniae
E. Salmonella enterica (Correct Answer)

Explanation: ***Salmonella enterica*** - Patients with **sickle cell disease** are particularly susceptible to **osteomyelitis** caused by *Salmonella* species due to impaired splenic function and increased gut permeability. - The presentation of **fever**, **painful swelling**, and **point tenderness over a bone** in a patient with a history of sickle cell disease strongly points towards *Salmonella* osteomyelitis. *Coccidioides immitis* - This fungus is a cause of **coccidioidomycosis**, common in **southwestern US desert regions**, but osteomyelitis is a less common manifestation and typically affects immunocompromised individuals more severely. - While it can cause osteomyelitis, it is not the most likely organism in a child with sickle cell disease presenting with acute osteomyelitis. *Pseudomonas aeruginosa* - **Pseudomonas osteomyelitis** is more commonly associated with puncture wounds (e.g., foot puncture through a shoe), intravenous drug use, or nosocomial infections. - While it can occur, it is less common as a primary cause of osteomyelitis in a child with sickle cell disease without these specific risk factors. *Escherichia coli* - *E. coli* can cause osteomyelitis, often in cases of **urinary tract infections** or **intra-abdominal infections** that spread hematogenously, or in settings of open fractures or surgical contamination. - It is not as classically associated with sickle cell disease-related osteomyelitis as *Salmonella*. *Streptococcus pneumoniae* - *S. pneumoniae* can cause osteomyelitis, especially in **young children** and individuals with **impaired immunity**, but it is generally less common than *Staphylococcus aureus* or *Salmonella* in sickle cell patients. - While sickle cell patients are prone to **pneumococcal infections**, *Salmonella* is a more specific and well-known cause of osteomyelitis in this population.

Question 40: A young infant is brought to an immunologist because of recurrent infections, which have not resolved despite appropriate medical treatment. On reviewing her medical history, the immunologist notes that the child has had frequent disseminated mycobacterial infections. He suspects a possible immunodeficiency. What is the most likely cause of this patient's immunodeficiency?

A. ATM gene defect
B. LFA-1 integrin defect
C. B-cell maturation defect
D. Interferon-gamma signaling defect (Correct Answer)
E. BTK gene defect

Explanation: ***Interferon-gamma signaling defect*** - **Interferon-gamma (IFN-γ)** is crucial for activating macrophages to kill intracellular pathogens like mycobacteria. A defect in its signaling pathway leads to impaired macrophage function and severe, recurrent mycobacterial infections. - This condition is often referred to as **Mendelian susceptibility to mycobacterial diseases (MSMD)**. *ATM gene defect* - An **ATM gene defect** is associated with **ataxia-telangiectasia**, which primarily presents with cerebellar ataxia, telangiectasias, and immunodeficiency characterized by **recurrent sinopulmonary infections** and an increased risk of lymphomas, not typically disseminated mycobacterial infections. - The immunodeficiency in ataxia-telangiectasia often involves **T-cell and IgA deficiency**. *LFA-1 integrin defect* - A defect in **LFA-1 integrin** causes **leukocyte adhesion deficiency type 1 (LAD-1)**, characterized by recurrent bacterial infections, impaired wound healing, delayed umbilical cord separation, and **leukocytosis**. - The primary defect is in leukocyte extravasation and adhesion, leading to impaired pus formation, not specifically disseminated mycobacterial disease. *B-cell maturation defect* - A **B-cell maturation defect** typically leads to **antibody deficiencies**, resulting in recurrent infections, particularly with **encapsulated bacteria** (e.g., Streptococcus pneumoniae, Haemophilus influenzae). - It would not primarily manifest as recurrent disseminated mycobacterial infections, as macrophage-mediated immunity is more critical for mycobacteria. *BTK gene defect* - A **BTK gene defect** causes **X-linked agammaglobulinemia (XLA)**, characterized by a virtual absence of B cells and severe deficiency of all immunoglobulin classes. - Patients present with recurrent bacterial infections starting around 6 months of age, once maternal antibodies wane, but **disseminated mycobacterial infections** are not the hallmark.

Question 41: An 18-year-old male in his first year of college presents to the emergency room with a fever and a severe headache. He reports having unprotected sex with several partners over the past few weeks. Upon examination, the male demonstrates nuchal rigidity and photophobia. His past medical history is notable for a lack of vaccinations beginning from infancy due to his parents' belief that vaccinations may cause autism. The bacteria causing these symptoms would most likely demonstrate which of the following?

A. Negative catalase test
B. Gram-negative bacteria (Correct Answer)
C. Urease positive
D. Positive quellung reaction
E. Lactose fermentation

Explanation: ***Gram-negative bacteria*** - The clinical picture of fever, severe headache, **nuchal rigidity**, and photophobia in an unvaccinated 18-year-old college student strongly suggests **bacterial meningitis** caused by *Neisseria meningitidis*. - *Neisseria meningitidis* is a **Gram-negative diplococcus**, which is the most definitive laboratory characteristic for identifying this organism. - College dormitory settings and unvaccinated status are major risk factors for **meningococcal meningitis**, and Gram stain is typically the first diagnostic step showing Gram-negative diplococci in CSF. *Positive quellung reaction* - The Quellung reaction (capsular swelling) is classically associated with **Streptococcus pneumoniae**, not *Neisseria meningitidis*. - While *N. meningitidis* does have a polysaccharide capsule, the Quellung test is not the standard identification method for this organism. - *S. pneumoniae* would be more common in older adults or those with specific risk factors like asplenia. *Negative catalase test* - *Neisseria meningitidis* is **catalase-positive**, so a negative catalase test would rule out this organism. - Catalase-negative organisms include Streptococcus and Enterococcus species, which have different clinical presentations. *Lactose fermentation* - *Neisseria meningitidis* is a **non-lactose fermenter** and does not utilize lactose fermentation for energy. - Lactose fermentation is characteristic of enteric Gram-negative bacteria like *E. coli* and *Klebsiella*, not Neisseria species. - *N. meningitidis* ferments **maltose and glucose**, which distinguishes it from *N. gonorrhoeae* (glucose only). *Urease positive* - *Neisseria meningitidis* is **urease-negative**, so urease positivity would rule out this organism. - Urease-positive bacteria include *Helicobacter pylori*, *Proteus* species, and *Klebsiella*, none of which typically cause meningitis in this clinical setting.

Question 42: A 15-year-old boy is brought to the emergency department by ambulance after his mother found him having muscle spasms and stiffness in his room. His mother stated he scraped his foot on a rusty razor on the bathroom floor 2 days prior. On presentation, his temperature is 102.0°F (38.9°C), blood pressure is 108/73 mmHg, pulse is 122/min, and respirations are 18/min. On physical exam, he is found to have severe muscle spasms and rigid abdominal muscles. In addition, he has a dirty appearing wound on his right foot. The patient's mother does not recall any further vaccinations since age 12. Finally, he is found to have difficulty opening his mouth so he is intubated. Which of the following treatment(s) should be provided to this patient?

A. Wound debridement and antitoxin
B. Antitoxin
C. Wound debridement
D. Wound debridement and booster vaccine
E. Wound debridement, antitoxin, and booster vaccine (Correct Answer)

Explanation: ***Wound debridement, antitoxin, and booster vaccine*** - The patient presents with classic symptoms of **tetanus** (muscle spasms, stiffness, trismus, fever) following a contaminated wound, and an uncertain vaccination history. - **Wound debridement** removes the source of toxin production, **antitoxin** (tetanus immune globulin) neutralizes circulating toxin, and a **booster vaccine** provides active immunity against future infections. *Wound debridement and antitoxin* - While **wound debridement** and **antitoxin** are critical for acute management, omitting the booster vaccine leaves the patient vulnerable to future tetanus infections. - A booster dose is essential to stimulate the patient's own immune system and provide **long-term immunity**, especially with a history of unknown vaccination status. *Antitoxin* - Administering only **antitoxin** would neutralize circulating toxins but would not address the ongoing production of toxins from the contaminated wound. - It also wouldn't provide **active immunization** to protect against future exposures. *Wound debridement* - **Wound debridement** alone removes the bacterial source but does not neutralize the already circulating **tetanus toxin**, which is responsible for the severe neurological symptoms. - It also fails to provide immediate passive immunity with antitoxin or active immunization with a booster. *Wound debridement and booster vaccine* - This option correctly addresses removing the source and providing active immunity but critically misses the immediate need for **antitoxin** to neutralize existing toxins and alleviate the life-threatening symptoms. - The **tetanus toxin** acts rapidly, and prompt neutralization is crucial to prevent further neurological damage and improve prognosis.

Question 43: A 27-year-old woman visits your office with a 3-day complaint of fever, malaise, myalgias, and headaches associated with vulvar itching, vulvar soreness, dysuria, and urethral discharge. The pain when urinating is so severe that she has attempted to avoid the use of the toilet. She just returned from a spring break in the tropics and explains that she had multiple unprotected sexual encounters with men and women. Upon physical examination, the temperature was 38.7°C (101.6°F) and the heart rate was 90/min. The pelvic examination revealed vulvar ulcers, vulvar excoriations, erythema, vaginal mucosa edema, and ulcerative cervicitis. Which of the following will best aid you in the identification of the specific organism causing the above symptoms?

A. Location of the lesions
B. Culture in Thayer-Martin agar
C. Direct fluorescence antigen (Correct Answer)
D. Serology
E. Tzanck smear

Explanation: ***Direct fluorescence antigen*** - A **direct fluorescence antigen (DFA)** test can rapidly identify viral antigens from the lesions, specifically for **Herpes Simplex Virus (HSV)**, which is highly suspected given the patient's symptoms (fever, malaise, myalgias, vulvar ulcers, dysuria, and recent unprotected sexual encounters). - **HSV** is a common cause of genital ulcers, and DFA offers a quick, sensitive, and specific method for detection directly from clinical samples. *Location of the lesions* - While the **location of the lesions** (vulvar ulcers) is characteristic of several sexually transmitted infections (STIs), it is not specific enough to identify the *specific organism* without further laboratory testing. - Conditions like syphilis, chancroid, and HSV all cause genital ulcers, making location alone insufficient for definitive diagnosis. *Culture in Thayer-Martin agar* - **Thayer-Martin agar** is selectively used for culturing **Neisseria gonorrhoeae**, which causes gonorrhea. - Although the patient has urethral discharge and dysuria, the presence of **vulvar ulcers** and systemic symptoms like fever and myalgias point away from uncomplicated gonorrhea. *Serology* - **Serology** detects antibodies to pathogens, indicating past or present infection, but is often less useful for identifying the *acute* causative organism in the initial stages of a symptomatic outbreak like this one. - For instance, HSV serology can distinguish between HSV-1 and HSV-2 exposure but does not confirm active infection in the way direct antigen methods or PCR do. *Tzanck smear* - A **Tzanck smear** can reveal characteristic **multinucleated giant cells** and **intranuclear inclusions**, which are indicative of herpesvirus infections (HSV or VZV). - However, it is less sensitive and specific than a direct fluorescence antigen test or PCR, and results can be variable depending on the quality of the smear and interpretation.

Question 44: An 11-year-old boy is brought to his pediatrician by his mother after he has complained of worsening left thumb pain for the last two weeks. The mother reports that the patient was previously healthy. Approximately 2 weeks ago, the family cat bit the patient’s thumb. The area around the bite wound then became red, hot, and slightly swollen and never healed. Earlier this week, the patient also started developing fevers that were recorded at home to be as high as 103.6°F. On exam, the patient's temperature is 102.2°F (39.0°C), blood pressure is 112/72 mmHg, pulse is 92/min, and respirations are 14/min. The patient’s left thumb is tender to touch over the proximal phalanx and the interphalangeal joint, but there is no obvious erythema or swelling. A radiograph performed in clinic is concerning for osteomyelitis at the proximal phalanx. Which of the following is the most likely cause of this patient’s condition?

A. Bartonella henselae
B. Pseudomonas aeruginosa
C. Eikenella corrodens
D. Pasteurella multocida (Correct Answer)
E. Salmonella spp.

Explanation: ***Pasteurella multocida*** - This patient presents with an **infected cat bite** with signs of **osteomyelitis**, making *Pasteurella multocida* the most likely causative organism. - *Pasteurella multocida* is a common organism found in the **oral flora of cats and dogs** and is frequently responsible for rapid onset infections following animal bites. *Bartonella henselae* - This bacterium is the causative agent of **cat scratch disease**, which typically presents as **lymphadenopathy** days to weeks after a cat scratch or bite. - While associated with cats, it's less likely to cause acute osteomyelitis directly following a bite in an immunocompetent child compared to *Pasteurella multocida*. *Pseudomonas aeruginosa* - This organism is more commonly associated with **puncture wounds through shoes** (especially involving the foot), leading to osteomyelitis, or infections in **immunocompromised individuals**. - While it can cause wound infections, it's not the most typical pathogen for an acute cat bite infection leading to osteomyelitis. *Eikenella corrodens* - *Eikenella corrodens* is part of the **normal human oral flora** and is a common cause of infections associated with **human bites** or "clenched-fist" injuries. - This scenario involves a cat bite, making *Eikenella corrodens* an unlikely primary pathogen. *Salmonella spp.* - *Salmonella* infections typically present as **gastroenteritis** and can sometimes lead to osteomyelitis, especially in individuals with **sickle cell disease**. - There is no history of preceding gastroenteritis or risk factors for *Salmonella* osteomyelitis in this patient, and it is not a typical pathogen for cat bite infections.

Question 45: A 14-year-old male is brought to the Emergency Department by his mother. She is worried because his face has become puffy and his urine has turned a tea-color. Patient history reveals the child recently suffered from a sore throat. The physician suspects a bacterial infection. Which of the following describes the likely bacteria responsible?

A. Gram negative
B. Bacitracin insensitive
C. Catalase positive
D. Beta-hemolytic (Correct Answer)
E. Coagulase positive

Explanation: ***Beta-hemolytic*** - The patient's symptoms (puffy face, tea-colored urine, recent sore throat) are classic for **post-streptococcal glomerulonephritis (PSGN)**, which is caused by a prior infection with **Group A Streptococcus (GAS)**. - **GAS** (Streptococcus pyogenes) is known for its **beta-hemolytic** activity, meaning it completely lyses red blood cells on blood agar, creating clear zones around colonies. *Gram negative* - **Group A Streptococcus** (Streptococcus pyogenes) are **Gram-positive cocci**, not Gram-negative. - Gram-negative bacteria have a different cell wall structure and typically cause different types of infections. *Bacitracin insensitive* - **Group A Streptococcus** (Streptococcus pyogenes) is typically **bacitracin sensitive**, meaning its growth is inhibited by bacitracin on a blood agar plate. - This characteristic is used in laboratory settings to differentiate GAS from other beta-hemolytic streptococci. *Catalase positive* - **Group A Streptococcus** (Streptococcus pyogenes) is **catalase negative**, meaning it does not produce the enzyme catalase. - **Staphylococcus species** are catalase-positive, which is a key differential test between *Staphylococcus* and *Streptococcus*. *Coagulase positive* - **Group A Streptococcus** (Streptococcus pyogenes) is **coagulase negative**. - **Staphylococcus aureus** is a notable coagulase-positive bacterium, and coagulase production is a significant virulence factor for this organism, not for GAS.

Question 46: A 24-year old G1P0 mother with no prenatal screening arrives to the hospital in labor and has an uneventful delivery. The infant is full term and has no significant findings on physical exam. Shortly after birth, an erythromycin ophthalmic ointment is applied to the newborn in order to provide prophylaxis against infection. Which of the following is the most common mechanism of resistance to the ointment applied to this newborn?

A. Methylation of 23S rRNA-binding site (Correct Answer)
B. Alteration of cell wall peptidoglycan
C. Increased efflux out of bacterial cells with plasmid-encoded transport pumps
D. Mutation in DNA polymerase
E. Penicillinase in bacteria cleaves the beta-lactam ring

Explanation: ***Methylation of 23S rRNA-binding site*** - Erythromycin is a **macrolide antibiotic** that inhibits bacterial protein synthesis by binding to the **23S ribosomal RNA** component of the 50S ribosomal subunit. - The most common mechanism of resistance to erythromycin in *Neisseria gonorrhoeae* (the primary target for prophylaxis) involves methylation of this binding site, which **prevents erythromycin from effectively binding** and exerts its action. *Alteration of amino acid cell wall* - This mechanism is not typical for resistance to erythromycin or other macrolides, as their primary target is **bacterial ribosomes**, not the cell wall. - Alterations in the cell wall are more commonly associated with resistance to antibiotics like **beta-lactams** (e.g., penicillin) that target cell wall synthesis. *Increased efflux out of bacterial cells with plasmid-encoded transport pumps* - While efflux pumps are a known resistance mechanism for various antibiotics, including macrolides, they are **not the most common or primary mechanism** for erythromycin resistance in *Neisseria gonorrhoeae*. - **MefA** and **mrsA** efflux pumps can lead to macrolide resistance, but ribosomal modification is more prevalent. *Mutation in DNA polymerase* - Mutations in **DNA polymerase** would typically affect DNA replication and repair, leading to resistance to antibiotics that target these processes, such as **fluoroquinolones**. - Erythromycin does not inhibit DNA polymerase; its mechanism of action is on **protein synthesis**. *Penicillinase in bacteria cleaves the beta-lactam ring* - **Penicillinase** is an enzyme that specifically **cleaves the beta-lactam ring** of penicillin and other beta-lactam antibiotics, rendering them inactive. - Erythromycin is a **macrolide**, not a beta-lactam antibiotic, and is therefore not affected by penicillinase.

Question 47: An 87-year-old male nursing home resident is currently undergoing antibiotic therapy for the treatment of a decubitus ulcer. One week into the treatment course, he experiences several episodes of watery diarrhea. Subsequent sigmoidoscopy demonstrates the presence of diffuse yellow plaques on the mucosa of the sigmoid colon. Which of the following is the best choice of treatment for this patient?

A. Intravenous vancomycin
B. Intravenous gentamicin
C. Oral metronidazole (Correct Answer)
D. Oral trimethoprim/sulfamethoxazole
E. Oral morphine

Explanation: ***Oral metronidazole*** - The patient's presentation with **watery diarrhea** and **yellow plaques (pseudomembranes) on sigmoidoscopy** after antibiotic therapy is classic for **Clostridioides difficile infection (CDI)**. - Among the options provided, **oral metronidazole** is the best choice as it achieves therapeutic concentrations in the colonic lumen and has activity against C. difficile. - Current **IDSA guidelines** recommend oral **vancomycin or fidaxomicin** as first-line therapy for CDI; however, metronidazole remains an acceptable alternative, particularly in resource-limited settings or when first-line agents are unavailable. - Metronidazole has good **colonic penetration** when administered orally and is effective against anaerobic bacteria including C. difficile. *Intravenous vancomycin* - While **vancomycin** is highly effective against C. difficile, it **must be administered orally** to treat CDI because IV vancomycin does not achieve adequate concentrations in the gut lumen. - Intravenous vancomycin is excreted primarily by the kidneys and does not reach the colonic mucosa in therapeutic amounts. - IV vancomycin is appropriate for systemic infections like **MRSA bacteremia or endocarditis**, but not for intestinal infections like CDI. *Intravenous gentamicin* - **Gentamicin** is an aminoglycoside antibiotic effective against **gram-negative bacteria** but has **no activity against C. difficile**, which is a gram-positive anaerobic bacillus. - Aminoglycosides carry significant risks of **nephrotoxicity and ototoxicity**, making them inappropriate for this clinical scenario. - Use of gentamicin would not address the underlying CDI and could worsen outcomes. *Oral trimethoprim/sulfamethoxazole* - **Trimethoprim/sulfamethoxazole** is a broad-spectrum antibiotic effective for various infections (UTIs, Pneumocystis, etc.) but has **no significant activity against C. difficile**. - Continued antibiotic use with agents ineffective against C. difficile could further disrupt normal gut flora and potentially **worsen the CDI**. *Oral morphine* - **Morphine** is an opioid analgesic with **no antibacterial properties** and therefore cannot treat bacterial infections like CDI. - Opioids can actually **slow gastrointestinal motility**, which may worsen outcomes in CDI by prolonging exposure to toxins. - While it might provide symptomatic relief of abdominal discomfort, it does not address the underlying infection and is contraindicated in infectious diarrhea.

Question 48: A 4-year-old male presents to the pediatrician with a one week history of fever, several days of bloody diarrhea, and right-sided abdominal pain. The mother explains that several other children at his son's pre-K have been having similar symptoms. She heard the daycare owner had similar symptoms and may have her appendix removed, but the mother claims this may just have been a rumor. Based on the history, the pediatrician sends for an abdominal ultrasound, which shows a normal vermiform appendix. She then sends a stool sample for culturing. The cultures demonstrate a Gram-negative bacteria that is motile at 25 C but not at 37 C, non-lactose fermenter, and non-hydrogen sulfide producer. What is the most likely causative agent?

A. Clostridium perfringens
B. Enterotoxigenic E. coli
C. Yersinia enterocolitica (Correct Answer)
D. Vibrio cholerae
E. Rotavirus

Explanation: ***Correct: Yersinia enterocolitica*** - The patient's symptoms (fever, bloody diarrhea, right-sided abdominal pain suggestive of **pseudoappendicitis**) combined with the **epidemiological link** to a daycare outbreak are classic for *Yersinia enterocolitica*. - The microbiology results are pathognomonic: Gram-negative, **motile at 25°C but not at 37°C** (temperature-dependent motility is the key distinguishing feature), non-lactose fermenter, and non-H2S producer. - *Yersinia enterocolitica* characteristically causes **mesenteric adenitis** mimicking appendicitis, particularly in children. *Incorrect: Clostridium perfringens* - Causes **food poisoning** (watery diarrhea, cramping) and **gas gangrene** (clostridial myonecrosis). - It is a **Gram-positive, spore-forming anaerobic bacillus**, which contradicts the Gram-negative finding in this case. - Does not typically present with pseudoappendicitis or fever lasting one week. *Incorrect: Enterotoxigenic E. coli (ETEC)* - Primarily causes **watery diarrhea** ("traveler's diarrhea") through heat-labile and heat-stable enterotoxins. - Does not typically cause bloody diarrhea, prolonged fever, or pseudoappendicitis. - While Gram-negative and motile, ETEC is motile at both temperatures and is a **lactose fermenter**, unlike the organism described. *Incorrect: Vibrio cholerae* - Causes **severe watery diarrhea** ("rice-water stools") with rapid dehydration through cholera toxin. - Associated with contaminated water in endemic areas, not daycare outbreaks. - Does not cause bloody diarrhea or pseudoappendicitis. - While Gram-negative and motile, it is motile at 37°C and produces a distinct clinical picture of massive fluid loss. *Incorrect: Rotavirus* - Common cause of **viral gastroenteritis** in children with fever, vomiting, and watery diarrhea. - It is a **virus** (not bacteria), so it would not grow on bacterial culture media and would not have Gram stain characteristics. - Does not cause bloody diarrhea or pseudoappendicitis syndrome.

Question 49: A 22-year-old sexually active female presents to the emergency department in severe pain. She states that she has significant abdominal pain that seems to worsen whenever she urinates. This seems to have progressed over the past day and is accompanied by increased urge and frequency. The emergency room physician obtains a urinalysis which demonstrates the following: SG: 1.010, Leukocyte esterase: Positive, Protein: Trace, pH: 7.5, RBC: Negative. Nitrite: Negative. A urease test is performed which is positive. What is the most likely cause of UTI in this patient?

A. Escherichia coli
B. Proteus mirabilis
C. Klebsiella pneumoniae
D. Staphylococcus saprophyticus (Correct Answer)
E. Serratia marcescens

Explanation: ***Staphylococcus saprophyticus*** - This patient's presentation with acute abdominal pain, dysuria, increased urinary urgency and frequency in a sexually active young female, combined with a **positive leukocyte esterase** on urinalysis and a **positive urease test**, is highly suggestive of **_Staphylococcus saprophyticus_**. - **_Staphylococcus saprophyticus_** is a common cause of **urinary tract infections (UTIs)** in young, sexually active women, and it is known to produce **urease**, which explains the positive urease test. *Escherichia coli* - While _E. coli_ is the most common cause of UTIs, accounting for 80-90% of cases, the **negative nitrite test** in this patient makes _E. coli_ less likely, as _E. coli_ is a **nitrate-reducing bacterium**. - A positive urease test is not characteristic of _E. coli_, as it is typically **urease-negative**. *Proteus mirabilis* - _Proteus mirabilis_ is a **urease-positive** bacterium and can cause UTIs, often associated with struvite stones and an alkaline urine pH. - However, _Proteus mirabilis_ typically produces **nitrite** (being a nitrate-reducer), which is negative in this patient's urinalysis, making it less likely. *Klebsiella pneumoniae* - _Klebsiella pneumoniae_ can cause UTIs and is often **urease-positive** but is more commonly associated with hospital-acquired infections or in patients with indwelling catheters. - Similar to _E. coli_ and _Proteus mirabilis_, _Klebsiella pneumoniae_ is a **nitrate-reducing bacterium** and would typically result in a positive nitrite test. *Serratia marcescens* - _Serratia marcescens_ can cause UTIs, particularly in hospitalized or immunocompromised patients. - It is typically **nitrite-positive** (a nitrate-reducer) and **urease-negative**, which does not align with the patient's urinalysis and positive urease test.

Question 50: A 20-year-old man presents to his physician with diarrhea, vomiting, and fever for the past 2 days. After laboratory evaluation, he is diagnosed with bacterial gastroenteritis. The man is a microbiology major and knows that the human gastrointestinal tract, respiratory tract, and skin are lined by epithelia which act as a barrier against several infective microorganisms. He also knows that there are specific T cells in these epithelia that play a part in innate immunity and in recognition of microbial lipids. Which of the following types of T cells is the man thinking of?

A. Naïve T cells
B. γδ T cells (Correct Answer)
C. Regulatory T cells
D. Natural killer T cells
E. Αβ T cells

Explanation: ***γδ T cells*** - **γδ T cells** are a distinct subset of T lymphocytes predominantly found in epithelial tissues (e.g., gut, skin, lung) and play a crucial role in **innate immunity** at barrier surfaces. - They are known for their ability to recognize and respond to conserved microbial antigens, including **lipids**, without prior major histocompatibility complex (MHC) presentation, making them key sentinels against pathogens. *Naïve T cells* - **Naïve T cells** are T cells that have not yet encountered their specific antigen and require presentation by MHC molecules for activation. - They circulate in lymphoid organs and blood, but are not typically enriched in epithelial barriers for immediate innate immune responses. *Regulatory T cells* - **Regulatory T cells (Tregs)** are primarily involved in **immune tolerance** and suppressing immune responses to prevent autoimmunity and excessive inflammation. - While important for maintaining gut homeostasis, their main role is not direct pathogen recognition or innate immunity against microbial lipids. *Natural killer T cells* - **Natural killer T (NKT) cells** are a heterogeneous group of T cells that express markers of both T cells and NK cells, and recognize **lipid antigens** presented by **CD1d molecules**. - Although they recognize lipids, NKT cells are typically found in the liver, spleen, and bone marrow, and are less abundant and less specialized for epithelial barrier defense than γδ T cells. *Αβ T cells* - **Αβ T cells** constitute the vast majority of T cells and recognize peptide antigens presented by **MHC class I and II molecules**. - While crucial for adaptive immunity, they do not directly recognize microbial lipids as a primary mechanism and are not specialized for innate immunity in epithelial tissues in the same way γδ T cells are.

Question 51: A 27-year-old man presents to the emergency department with unrelenting muscle spasms for the past several hours. The patient’s girlfriend states that he started having jaw spasms and soreness last night but now his neck, back, and arms are spasming. She also states that he stepped on a nail about 1 week ago. Past medical history is noncontributory. The patient's vaccination status is unknown at this time. Today, the vital signs include temperature 39.1°C (102.4°F), heart rate 115/min, blood pressure 145/110 mm Hg, and respiratory rate 10/min. On exam, the patient is in obvious discomfort, with a clenched jaw and extended neck. Labs are drawn and a basic metabolic panel comes back normal and the white blood cell (WBC) count is moderately elevated. Which of the following is the most likely etiology of this patient’s symptoms?

A. An exotoxin that causes ADP-ribosylation of EF-2
B. An edema factor that functions as adenylate cyclase
C. A heat-labile toxin that inhibits ACh release at the NMJ
D. An exotoxin that cleaves SNARE proteins (Correct Answer)
E. A toxin that disables the G-protein coupled receptor

Explanation: ***An exotoxin that cleaves SNARE proteins*** - The patient's presentation with **unrelenting muscle spasms**, jaw spasms (**trismus**), extended neck (**opisthotonus**), and a recent **puncture wound** are classic signs of **tetanus**. - **Tetanospasmin**, the neurotoxin produced by *Clostridium tetani*, acts by cleaving **SNARE proteins**, which are essential for the release of **inhibitory neurotransmitters** (glycine and GABA) from spinal interneurons, leading to uncontrolled muscle contraction. *An exotoxin that causes ADP-ribosylation of EF-2* - This mechanism describes **diphtheria toxin**, which is produced by *Corynebacterium diphtheriae* and inhibits protein synthesis in eukaryotic cells. - While *C. diphtheriae* can cause systemic effects, it primarily manifests as **upper respiratory tract infection** with pseudomembrane formation, lymphadenopathy, and myocarditis, not generalized muscle spasms. *An edema factor that functions as adenylate cyclase* - This describes the **edema factor** component of **anthrax toxin**, produced by *Bacillus anthracis*. - Anthrax typically causes cutaneous, inhalational, or gastrointestinal infections, and its symptoms do not include the generalized muscle spasms seen in this patient. *A heat-labile toxin that inhibits ACh release at the NMJ* - This mechanism describes **botulinum toxin**, produced by *Clostridium botulinum*, which causes **flaccid paralysis** by preventing the release of acetylcholine at the neuromuscular junction. - The patient exhibits muscle spasms and rigidity (**spastic paralysis**), which is directly opposite to the effects of botulinum toxin. *A toxin that disables the G-protein coupled receptor* - While various toxins can affect G-protein coupled receptors (e.g., cholera toxin or pertussis toxin), this general description does not specifically match the clinical presentation of tetanus. - Toxins affecting G-protein coupled receptors typically lead to symptoms like **severe diarrhea** (cholera) or **whooping cough** (pertussis) rather than generalized muscle spasms.

Question 52: A previously healthy 3-year-old boy is brought to the physician by his mother because of a headache, fever, and facial pain that started 10 days ago. The symptoms initially improved but have gotten significantly worse over the past 2 days. Immunizations are up-to-date. His temperature is 39.1°C (102.3°F). Physical examination shows tenderness to palpation over both cheeks. Gram stain of a nasal swab shows small, gram-negative coccobacilli. Which of the following most likely accounts for this patient's infection with the causal pathogen?

A. Causal pathogen is unencapsulated (Correct Answer)
B. Causal pathogen produces phospholipase C
C. Host has hyperviscous secretions
D. Causal pathogen expresses protein A
E. Host has impaired splenic opsonization

Explanation: ***Correct: Causal pathogen is unencapsulated*** - This patient's symptoms (headache, fever, facial pain, tenderness over cheeks, "double sickening" phenomenon) are highly suggestive of **acute bacterial sinusitis**, likely caused by *Haemophilus influenzae*. - Despite being immunized, the disease in a young, vaccinated child suggests an **unencapsulated, non-typeable strain** of *H. influenzae*, which is not covered by the standard **Hib vaccine**. - The Hib vaccine protects against type b (encapsulated) strains, but non-typeable *H. influenzae* (NTHi) lacks a polysaccharide capsule and therefore is not prevented by vaccination. *Incorrect: Causal pathogen produces phospholipase C* - **Phospholipase C** is a virulence factor primarily associated with bacteria like *Pseudomonas aeruginosa* and *Clostridium perfringens*, which cause different types of infections (e.g., necrotizing fasciitis, gas gangrene). - It is not a characteristic virulence factor of *Haemophilus influenzae* and is not relevant to sinusitis. *Incorrect: Host has hyperviscous secretions* - **Hyperviscous secretions** are a hallmark of **cystic fibrosis**, a genetic condition primarily affecting the lungs and pancreas, leading to recurrent respiratory infections. - There is no information in the vignette to suggest cystic fibrosis, and the patient's presentation is more typical of acute sinusitis in an immunocompetent child. *Incorrect: Causal pathogen expresses protein A* - **Protein A** is a virulence factor produced by *Staphylococcus aureus*, which binds to the Fc region of antibodies, preventing opsonization and phagocytosis. - While *S. aureus* can cause sinusitis, the Gram stain showing **small, gram-negative coccobacilli** does not align with *S. aureus*, which are gram-positive cocci in clusters. *Incorrect: Host has impaired splenic opsonization* - **Impaired splenic opsonization** (due to asplenia or functional asplenia) increases susceptibility to infections by **encapsulated bacteria** such as *Streptococcus pneumoniae*, *Haemophilus influenzae* type b (Hib), and *Neisseria meningitidis*. - The patient has received immunizations, implying protection against encapsulated strains covered by vaccines, and the presentation points to a non-typeable, unencapsulated strain rather than a problem with splenic function.

Question 53: A 23-year-old man presents to student health for a cough. The patient states he has paroxysms of coughing followed by gasping for air. The patient is up to date on his vaccinations and is generally healthy. He states he has felt more stressed lately secondary to exams. His temperature is 101.0°F (38.3°C), blood pressure is 125/65 mmHg, pulse is 105/min, respirations are 14/min, and oxygen saturation is 98% on room air. Laboratory values are notable for the findings below. Hemoglobin: 12 g/dL Hematocrit: 36% Leukocyte count: 13,500/mm^3 with a lymphocytosis Platelet count: 197,000/mm^3 Physical exam is notable for clear breath sounds bilaterally. Which of the following is the best next step in management?

A. Azithromycin (Correct Answer)
B. PCR for Bordetella pertussis
C. Chest radiograph
D. Culture
E. Penicillin

Explanation: ***Azithromycin*** - This patient's symptoms (paroxysmal cough followed by gasping), fever, and **lymphocytosis**, despite being vaccinated, are highly suggestive of **pertussis** (whooping cough). - **Macrolide antibiotics** like azithromycin are the recommended treatment for pertussis, as they can reduce the duration and severity of symptoms and prevent transmission, especially when given early in the disease course. *PCR for Bordetella pertussis* - While a **PCR test** would confirm the diagnosis, the prompt asks for the **best next step in management**, implying treatment rather than diagnosis given the clear clinical picture. - Due to the highly contagious nature of pertussis, treatment should ideally be initiated promptly based on clinical suspicion, especially within the **catarrhal** or early **paroxysmal stage**, without waiting for PCR results. *Chest radiograph* - A chest radiograph is generally **not indicated** for uncomplicated pertussis, as clear breath sounds are noted and it is usually a clinical diagnosis. - It would be more relevant to rule out complications like **pneumonia**, which is not immediately suggested by the given information. *Culture* - **Bacterial culture** for *Bordetella pertussis* from a nasopharyngeal swab is a diagnostic tool, but it is **less sensitive** and **takes longer** to yield results compared to PCR. - Given the urgency for treatment to reduce transmission and symptoms, culture is not the most appropriate *next step in management*. *Penicillin* - Penicillin is **not effective** against *Bordetella pertussis* as *B. pertussis* is **a** Gram-negative bacterium that is inherently resistant to penicillins. - **Macrolide antibiotics** are the drug class of choice for pertussis due to their efficacy against this organism.

Question 54: A 19-year-old male is found to have Neisseria gonorrhoeae bacteremia. This bacterium produces an IgA protease capable of cleaving the hinge region of IgA antibodies. What is the most likely physiological consequence of such a protease?

A. Membrane attack complex formation is impaired
B. Impaired antibody binding to mast cells
C. Opsonization and phagocytosis of pathogen cannot occur
D. Impaired adaptive immune system memory
E. Impaired mucosal immune protection (Correct Answer)

Explanation: ***Impaired mucosal immune protection*** - **IgA** is the primary antibody mediating **mucosal immunity**, protecting surfaces like the urogenital tract from pathogens. - Cleavage of IgA by a protease directly compromises its ability to bind to and neutralize pathogens at these mucosal surfaces, facilitating infection. *Membrane attack complex formation is impaired* - The **membrane attack complex (MAC)** is primarily formed by components of the **complement system (C5b-C9)**, which is activated by IgG and IgM, not IgA. - While IgA can activate the alternative pathway of complement, its primary role is not in MAC formation. *Impaired antibody binding to mast cells* - **Mast cells** primarily bind **IgE antibodies** via their Fc receptors, leading to degranulation upon allergen binding. - IgA does not typically bind to mast cells, so IgA protease activity would not directly impact this process. *Opsonization and phagocytosis of pathogen cannot occur* - **Opsonization** leading to phagocytosis is predominantly mediated by **IgG antibodies** and **complement proteins (e.g., C3b)**. - While IgA can contribute to opsonization to some extent, it is not the primary mediator, and its impairment would not completely prevent all opsonization. *Impaired adaptive immune system memory* - **Adaptive immune system memory** is largely mediated by **memory B cells** and **memory T cells**, which produce and respond to various antibody isotypes (IgG, IgA, IgM, IgE). - The cleavage of existing IgA antibodies does not directly impair the generation or function of memory lymphocytes, although it might lead to more frequent infections requiring a new immune response.

Question 55: A patient is infected with a pathogen and produces many antibodies to many antigens associated with that pathogen via Th cell-activated B cells. This takes place in the germinal center of the lymphoid tissues. If the same patient is later re-infected with the same pathogen, the immune system will respond with a much stronger response, producing antibodies with greater specificity for that pathogen in a shorter amount of time. What is the term for this process that allows the B cells to produce antibodies specific to that antigen?

A. Affinity maturation (Correct Answer)
B. Avidity
C. Immunoglobulin class switching
D. T cell negative selection
E. T cell positive selection

Explanation: ***Affinity maturation*** - **Affinity maturation** is the process by which B cells produce antibodies with progressively higher affinity for an antigen over the course of an immune response, allowing for a more specific and potent response upon re-exposure. - This process occurs primarily in the **germinal centers** of lymphoid organs, driven by somatic hypermutation of antibody genes and subsequent selection of B cells exhibiting increased binding affinity. *Avidity* - **Avidity** refers to the overall strength of binding between a multivalent antibody and a multivalent antigen, taking into account the combined strength of multiple binding sites. - While high avidity is a characteristic of effective antibody responses, it describes the strength of binding rather than the *process* of improving specificity and affinity over time. *Immunoglobulin class switching* - **Immunoglobulin class switching** (or isotype switching) is the process by which B cells change the class of antibody they produce (e.g., from IgM to IgG, IgA, or IgE), while retaining the same antigen specificity. - This process diversifies the effector functions of antibodies but does not directly describe the *improvement in antigen binding affinity* or specificity. *T cell negative selection* - **T cell negative selection** is a critical process in the thymus where T cells that react too strongly to self-antigens are eliminated or inactivated to prevent autoimmunity. - This process is fundamental for establishing central tolerance in T cells and is separate from the B cell-mediated improvement in antibody specificity described. *T cell positive selection* - **T cell positive selection** also occurs in the thymus, ensuring that only T cells capable of recognizing self-MHC molecules survive and mature. - This process is essential for T cell function (MHC restriction) but is distinct from the described mechanism of B cell antibody refinement.

Question 56: A 3-year-old female is found to have unusual susceptibility to infections by catalase-producing organisms. This patient likely has a problem with the function of which of the following cell types?

A. Natural killer cells
B. Eosinophils
C. B cells
D. T cells
E. Neutrophils (Correct Answer)

Explanation: ***Neutrophils*** - Catalase-producing organisms, such as *Staphylococcus aureus* or *Aspergillus*, are typically cleared by **phagocytic cells**, specifically **neutrophils**, which use the **respiratory burst** to produce reactive oxygen species. - A defect in neutrophil function, particularly in the **NADPH oxidase enzyme complex** responsible for the respiratory burst, leads to **chronic granulomatous disease (CGD)**, characterized by increased susceptibility to infections by these specific pathogens. *Natural killer cells* - **Natural killer (NK) cells** are primarily involved in the anti-viral and anti-tumor immune responses, recognizing and **killing infected or malignant cells**. - They do not play a primary role in clearing bacterial or fungal infections, especially those caused by catalase-producing organisms. *Eosinophils* - **Eosinophils** are primarily involved in defense against **parasitic infections** and in mediating **allergic reactions**. - Their role in clearing common bacterial or fungal infections is limited. *B cells* - **B cells** are responsible for **humoral immunity**, producing **antibodies** that neutralize pathogens and toxins or opsonize them for phagocytosis. - While antibodies can aid in the clearance of many pathogens, a primary deficiency in B cell function (e.g., agammaglobulinemia) would lead to broad susceptibility to encapsulated bacteria, not specifically catalase-positive organisms. *T cells* - **T cells** are central to **cell-mediated immunity**, recognizing and eliminating intracellular pathogens or directly killing infected cells. - Deficiencies in T cell function (e.g., SCID) lead to severe immunodeficiency with susceptibility to opportunistic infections, but do not specifically point to problems with catalase-producing organisms as the hallmark.

Question 57: A 28-year-old woman comes to the physician because of a 4-day history of lower abdominal pain and pain with urination. Five months ago, she was treated for gonococcal urethritis. She recently moved in with her newlywed husband. She is sexually active with her husband and they do not use condoms. Her only medication is an oral contraceptive. Her temperature is 37.5°C (99.7°F) and blood pressure is 120/74 mm Hg. There is tenderness to palpation over the pelvic region. Pelvic examination shows a normal-appearing vulva and vagina. Laboratory studies show: Leukocyte count 8,400/mm3 Urine pH 6.7 Protein trace WBC 60/hpf Nitrites positive Bacteria positive Which of the following is the most likely causal organism?

A. Enterococcus faecalis
B. Klebsiella pneumoniae
C. Neisseria gonorrhoeae
D. Escherichia coli (Correct Answer)
E. Staphylococcus saprophyticus

Explanation: ***Escherichia coli*** - The presence of **nitrites** in the urine analysis strongly suggests a urinary tract infection caused by a **nitrate-reducing bacterium**, such as *E. coli*. - *E. coli* is the **most common cause of uncomplicated UTIs**, especially in sexually active women, and the symptoms (lower abdominal pain, dysuria, pyuria, bacteriuria) are classic for a UTI. *Enterococcus faecalis* - While *Enterococcus faecalis* can cause UTIs, it is **less common** than *E. coli* in uncomplicated cases and typically **does not produce nitrites** in urine due to lacking nitrate reductase. - It is more commonly associated with UTIs in hospitalized patients or those with urinary tract abnormalities. *Klebsiella pneumoniae* - *Klebsiella pneumoniae* can cause UTIs and is a **nitrite-producing bacterium**, but it is a **less frequent cause** of community-acquired uncomplicated UTIs compared to *E. coli*. - It is more often associated with healthcare-associated infections or UTIs in compromised hosts. *Neisseria gonorrhoeae* - While the patient has a history of gonococcal urethritis, current symptoms are more consistent with a UTI, and *Neisseria gonorrhoeae* is a **rare cause of cystitis** or pyelonephritis. - Gonorrhea primarily causes urethritis, cervicitis, or pelvic inflammatory disease, and **does not typically produce nitrites** from nitrates in urine. *Staphylococcus saprophyticus* - *Staphylococcus saprophyticus* is a common cause of UTIs in young, sexually active women, but it is **nitrite-negative** because it does not possess nitrate reductase. - The positive nitrites in the urine make *E. coli* a more likely culprit in this case.

Question 58: A 12-year-old boy admitted to the intensive care unit 1 day ago for severe pneumonia suddenly develops hypotension. He was started on empiric antibiotics and his blood culture reports are pending. According to the nurse, the patient was doing fine until his blood pressure suddenly dropped. Vital signs include: blood pressure is 88/58 mm Hg, temperature is 39.4°C (103.0°F), pulse is 120/min, and respiratory rate is 24/min. His limbs feel warm. The resident physician decides to start him on intravenous vasopressors, as the blood pressure is not responding to intravenous fluids. The on-call intensivist suspects shock due to a bacterial toxin. What is the primary mechanism responsible for the pathogenesis of this patient's condition?

A. Inactivation of elongation factor (EF) 2
B. Inhibition of GABA and glycine
C. Inhibition of acetylcholine release
D. Release of tumor necrosis factor (TNF) (Correct Answer)
E. Degradation of lecithin in cell membranes

Explanation: ***Release of tumor necrosis factor (TNF)*** - The patient's presentation with **warm limbs** and **hypotension** despite fluid resuscitation in the setting of severe pneumonia is highly suggestive of **septic shock**. - **Bacterial toxins**, particularly **endotoxins** from gram-negative bacteria or **exotoxins** like superantigens, trigger a massive **inflammatory response** by stimulating immune cells to release pro-inflammatory cytokines such as **TNF-α**, IL-1, and IL-6, leading to systemic vasodilation and capillary leak. *Inactivation of elongation factor (EF) 2* - This is the mechanism of action of **diphtheria toxin** and **exotoxin A** from *Pseudomonas aeruginosa*. - While these toxins can cause severe systemic illness, their primary role is not typically the induction of septic shock characterized by widespread vasodilation and warm extremities. *Inhibition of GABA and glycine* - This mechanism is characteristic of **tetanus toxin**, which prevents the release of inhibitory neurotransmitters and leads to spastic paralysis. - This is not consistent with the patient's presentation of septic shock. *Inhibition of acetylcholine release* - This is the mechanism of action of **botulinum toxin**, which causes flaccid paralysis by blocking acetylcholine release at the neuromuscular junction. - This effect is not associated with the pathogenesis of septic shock. *Degradation of lecithin in cell membranes* - This mechanism is associated with **alpha toxin** of *Clostridium perfringens* (lecithinase), which causes gas gangrene and hemolysis. - While this toxin contributes to tissue damage in certain infections, it is not the primary mechanism behind the systemic inflammatory response and vasodilation seen in septic shock.

Question 59: A 32-year-old man comes to the emergency department because of nausea and vomiting for the past 2 hours. The patient has neither had diarrhea nor fever. Four hours ago he ate some leftover Indian rice dish he had ordered the night before. There is no history of serious illness. He immigrated from India 8 years ago with his family and now works as a butcher. He appears ill. His temperature is 36.7°C (98°F), pulse is 85/min, and blood pressure is 115/70 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 98%. Which of the following is the most likely causal organism?

A. Shigella dysenteriae
B. Bacillus cereus (Correct Answer)
C. Staphylococcus aureus
D. Vibrio parahaemolyticus
E. Enterohemorrhagic Escherichia coli

Explanation: ***Bacillus cereus*** - The patient's symptoms of rapid-onset nausea and vomiting (1-5 hours after ingestion) without diarrhea or fever, following consumption of a **rice dish**, are highly characteristic of the emetic toxin produced by *Bacillus cereus*. - This **cereulide toxin** is heat-stable and pre-formed in food, leading to quick onset, often associated with improperly stored or reheated rice. *Shigella dysenteriae* - This organism typically causes **bloody diarrhea**, fever, and severe abdominal cramps (bacillary dysentery), not primarily nausea and vomiting without diarrhea. - The incubation period is usually longer (24-72 hours) than the rapid onset seen in this patient. *Staphylococcus aureus* - While *S. aureus* can cause rapid-onset nausea and vomiting due to pre-formed **enterotoxins**, it is more commonly associated with protein-rich foods like deli meats, dairy products, and custards, rather than rice dishes. - The symptoms can be very similar to *Bacillus cereus* emetic type, but the food source points away from *S. aureus* in this specific scenario. *Vibrio parahaemolyticus* - This pathogen is associated with consumption of raw or undercooked **seafood** and typically causes watery diarrhea, abdominal cramps, nausea, vomiting, and fever. - The patient ate a rice dish, not seafood, and his primary symptoms are nausea and vomiting without diarrhea. *Enterohemorrhagic Escherichia coli* - EHEC typically causes severe **abdominal cramps**, **bloody diarrhea** (hemorrhagic colitis), and may lead to hemolytic uremic syndrome (HUS). - Nausea and vomiting can occur but are usually accompanied by bloody diarrhea, which is absent in this patient.

Question 60: A researcher is studying the effects of various substances on mature B-cells. She observes that while most substances are only able to promote the production of antibodies when the B-cells are co-cultured with T-cells, a small subset of substances are able to trigger antibody production even in the absence of T-cells. She decides to test these substances that stimulate B-cells alone by injecting them into model organisms. She then analyzes the characteristics of the response that is triggered by these substances. Which of the following correctly describes how the immune response triggered by the B-cell-alone-substances compares with that triggered by substances that also require T-cells?

A. Is T-dependent
B. Is T-independent (Correct Answer)
C. Requires MHC class II presentation
D. Requires cognate interaction
E. Requires B7-CD28 interaction

Explanation: ***Is T-independent*** - The scenario describes substances that activate B-cells and induce antibody production **without the need for T-cells**, which is the defining characteristic of a T-independent immune response. - T-independent activation typically involves **polysaccharide antigens** or other repetitive structures that can cross-link multiple B-cell receptors, providing a strong enough signal for activation without T-cell help. *Is T-dependent* - This option is incorrect because the question explicitly states that the substances can trigger antibody production **in the absence of T-cells**. - A T-dependent response requires **CD4+ T-helper cells** to activate B-cells through co-stimulation and cytokine signaling. *Requires MHC class II presentation* - While B-cells can present antigens via **MHC class II** to T-cells in T-dependent responses, T-independent activation of B-cells does not necessarily require antigen presentation on MHC class II molecules. - T-independent antigens often directly activate B-cells through **toll-like receptors (TLRs)** or extensive cross-linking of B-cell receptors. *Requires cognate interaction* - **Cognate interaction** refers to the specific recognition between an antigen-presenting B-cell and a helper T-cell, which is a hallmark of T-dependent responses. - Since the B-cells are producing antibodies in the absence of T-cells, cognate interaction is not required in this specific scenario. *Requires B7-CD28 interaction* - The **B7-CD28 interaction** is a crucial co-stimulatory signal provided by antigen-presenting cells (like B-cells) to T-cells, and from T-cells back to B-cells, in a T-dependent immune response. - As the scenario involves T-cell independent activation, this co-stimulatory pathway between T-cells and B-cells is not essential for antibody production.

Question 61: A 50-year-old male presents to the emergency room complaining of fever, shortness of breath, and diarrhea. He returned from a spa in the Rocky Mountains five days prior. He reports that over the past two days, he developed a fever, cough, dyspnea, and multiple watery stools. His past medical history is notable for major depressive disorder and peptic ulcer disease. He takes omeprazole and paroxetine. He does not smoke and drinks alcohol on social occasions. His temperature is 102.8°F (39.3°C), blood pressure is 120/70 mmHg, pulse is 65/min, and respirations are 20/min. Physical examination reveals dry mucus membranes, delayed capillary refill, and rales at the bilateral lung bases. A basic metabolic panel is shown below: Serum: Na+: 126 mEq/L Cl-: 100 mEq/L K+: 4.1 mEq/L HCO3-: 23 mEq/L Ca2+: 10.1 mg/dL Mg2+: 2.0 mEq/L Urea nitrogen: 14 mg/dL Glucose: 90 mg/dL Creatinine: 1.1 mg/dL Which of the following is the most appropriate growth medium to culture the pathogen responsible for this patient’s condition?

A. Eaton’s agar
B. Bordet-Gengou agar
C. Thayer-Martin agar
D. Charcoal yeast agar with iron and cysteine (Correct Answer)
E. Sorbitol-MacConkey agar

Explanation: ***Charcoal yeast agar with iron and cysteine*** - This patient's symptoms (fever, cough, dyspnea, diarrhea) after visiting a spa, combined with **hyponatremia** (Na+ 126 mEq/L), are highly characteristic of **Legionnaires' disease** caused by *Legionella pneumophila*. - *Legionella* is a fastidious organism that requires specialized media for growth, specifically charcoal yeast extract (BCYE) agar supplemented with **L-cysteine** and **iron salts**. *Eaton’s agar* - Eaton's agar is a specialized medium used for the primary isolation of **Mycoplasma pneumoniae**. - *Mycoplasma pneumoniae* typically causes **atypical pneumonia** but does not present with severe gastrointestinal symptoms or hyponatremia as seen in this patient. *Bordet-Gengou agar* - Bordet-Gengou agar, containing potato extract, glycerol, and blood, is the selective medium used for the isolation of **Bordetella pertussis**, the causative agent of whooping cough. - The clinical presentation of **pertussis** involves paroxysmal coughing fits, often with a 'whoop,' and is distinct from the patient's symptoms. *Thayer-Martin agar* - Thayer-Martin agar is a selective medium primarily used for the isolation of **Neisseria gonorrhoeae** and *Neisseria meningitidis*. - These bacteria cause gonorrhea and meningitis, respectively, and are not associated with the respiratory and gastrointestinal symptoms described. *Sorbitol-MacConkey agar* - Sorbitol-MacConkey agar is a differential and selective medium used to detect **enterohemorrhagic Escherichia coli O157:H7** in stool samples, which appears as colorless colonies because it cannot ferment sorbitol. - While the patient has diarrhea, the predominant respiratory symptoms, hyponatremia, and exposure history point away from E. coli O157:H7 as the primary pathogen.

Question 62: A 26-year-old woman presents to the emergency department with confusion, severe myalgia, fever, and a rash over her inner left thigh. The patient was diagnosed with pharyngitis three days ago and prescribed antibiotics, but she did not take them. Her blood pressure is 90/60 mm Hg, heart rate is 99/min, respiratory rate is 17/min, and temperature is 38.9°C (102.0°F). On physical examination, the patient is disoriented. The posterior wall of her pharynx is erythematous and swollen and protrudes into the pharyngeal lumen. There is a diffuse maculopapular rash over her thighs and abdomen. Which of these surface structures interacts with the causative agent of her condition?

A. Constant part of TCR α-chain
B. CD4
C. CD3
D. CD1
E. Variable part of TCR β-chain (Correct Answer)

Explanation: ***Variable part of TCR β-chain*** - The clinical presentation suggests **toxic shock syndrome (TSS)**, likely caused by **Staphylococcus aureus** or **Streptococcus pyogenes**, which produce **superantigens** (e.g., toxic shock syndrome toxin-1, streptococcal pyrogenic exotoxins). - Superantigens bind to the **variable beta chain (Vβ) of the T-cell receptor (TCR)** and the **MHC class II molecule** on antigen-presenting cells **outside the antigen-binding groove**, bypassing normal antigen processing. - This leads to **massive, non-specific T-cell activation** (up to 20% of T cells) and **cytokine storm** (IL-2, IFN-γ, TNF-α), resulting in the clinical features of TSS: fever, hypotension, diffuse rash, and multiorgan dysfunction. *Constant part of TCR α-chain* - The **constant region of the TCR α-chain** is involved in structural integrity and signal transduction but does not directly interact with superantigens. - Superantigens specifically target the **variable β chain** for cross-linking with MHC class II, not the constant α-chain. *CD4* - **CD4** is a co-receptor found on helper T cells that binds to **MHC class II molecules** during normal antigen presentation. - While superantigens interact with MHC class II, the primary TCR interaction site is the **Vβ region**, not the CD4 co-receptor itself. - CD4 plays a supportive role in TCR signaling but is not the direct binding target of superantigens. *CD3* - The **CD3 complex** (CD3γ, CD3δ, CD3ε, CD3ζ chains) associates with the TCR to form the complete **TCR-CD3 complex** and is essential for signal transduction upon antigen recognition. - While CD3 is crucial for T-cell activation signaling, it is not the direct binding site for superantigens, which specifically engage the **Vβ region**. *CD1* - **CD1 molecules** are MHC class I-like glycoproteins that present **lipid and glycolipid antigens** (not peptide antigens) to specialized T cells, including NKT cells. - CD1 is unrelated to the mechanism of superantigen binding, which involves **MHC class II** and the **variable beta chain of the TCR**.

Question 63: A 32-year-old man with hypertension and gout comes to the physician with left flank pain and bloody urine for two days. He does not smoke cigarettes but drinks two beers daily. Home medications include hydrochlorothiazide and ibuprofen as needed for pain. Physical examination shows left costovertebral angle tenderness. Urine dipstick is strongly positive for blood. Microscopic analysis of a stone found in the urine reveals a composition of magnesium ammonium phosphate. Which of the following is the strongest predisposing factor for this patient's condition?

A. Uric acid precipitation
B. Hereditary deficiency in amino acid reabsorption
C. Use of vitamin C supplements
D. Urinary tract infection (Correct Answer)
E. Ethylene glycol ingestion

Explanation: ***Urinary tract infection*** - The presence of a **magnesium ammonium phosphate stone**, also known as a **struvite stone**, is highly indicative of a urinary tract infection (UTI). These stones are formed in the presence of **urease-producing bacteria** (e.g., *Proteus mirabilis, Klebsiella pneumoniae*) that metabolize urea into ammonia, increasing localized pH and promoting crystal formation. - While the patient has multiple predisposing factors for other types of kidney stones (e.g., gout and hydrochlorothiazide for calcium and uric acid stones), the specific composition of the stone points directly to an underlying infection as the primary predisposing factor. *Uric acid precipitation* - This typically leads to **uric acid stones**, which are common in patients with **gout** and **hyperuricemia**. The stone described is magnesium ammonium phosphate, not uric acid. - Uric acid stones are usually radiolucent and are not directly associated with infection, unlike struvite stones. *Hereditary deficiency in amino acid reabsorption* - This condition, such as **cystinuria**, leads to the formation of **cystine stones**. These stones are hexagonal in shape and are caused by impaired tubular reabsorption of certain amino acids. - The described stone composition is magnesium ammonium phosphate, not cystine. *Use of vitamin C supplements* - Excessive vitamin C (ascorbic acid) intake can be metabolized to **oxalate**, potentially contributing to **calcium oxalate stone** formation, especially in susceptible individuals. - This patient's stone is a struvite stone, not a calcium oxalate stone. *Ethylene glycol ingestion* - Ethylene glycol poisoning is associated with the formation of **calcium oxalate monohydrate crystals** in the urine, which can lead to acute kidney injury and flank pain. - The stone composition here is magnesium ammonium phosphate, indicating a different etiology.

Question 64: A 72-year-old patient presents to the emergency department because of abdominal pain, diarrhea, and fever. He was started on levofloxacin for community-acquired pneumonia 2 weeks prior with resolution of his pulmonary symptoms. He has had hypertension for 20 years, for which he takes amlodipine. His temperature is 38.3°C (101.0°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. On examination, mild abdominal distension with minimal tenderness was found. Laboratory tests reveal a peripheral white blood cell count of 12.000/mm3 and a stool guaiac mildly positive for occult blood. Which of the following best describe the mechanism of this patient illness?

A. Damage to the gastrointestinal tract by enteropathogenic viruses
B. Autoimmune inflammation of the rectum
C. Disruption of normal bowel flora and infection by spore-forming rods (Correct Answer)
D. Decreased blood flow to the gastrointestinal tract
E. Presence of osmotically active, poorly absorbed solutes in the bowel lumen

Explanation: ***Disruption of normal bowel flora and infection by spore-forming rods*** - This describes **Clostridioides difficile infection (CDI)**, which is strongly suggested by the patient's recent antibiotic use (levofloxacin, a fluoroquinolone) followed by abdominal pain, diarrhea, fever, and leukocytosis. - Antibiotics disrupt the normal gut microbiome, allowing **C. difficile (spore-forming rods)** to proliferate and produce toxins that cause colitis. *Damage to the gastrointestinal tract by enteropathogenic viruses* - While viral gastroenteritis can cause these symptoms, the **recent history of antibiotic use** makes CDI a much more likely diagnosis. - Viral infections typically resolve spontaneously and are less likely to cause a significant **leukocytosis** and **occult blood in stool** in this context. *Autoimmune inflammation of the rectum* - Conditions like **ulcerative colitis**, an autoimmune disease, can cause similar symptoms but typically have a **chronic or relapsing course** and are not usually triggered by recent antibiotic use. - The acute presentation following antibiotics strongly points away from an autoimmune process. *Decreased blood flow to the gastrointestinal tract* - **Ischemic colitis** can cause abdominal pain and bloody diarrhea, especially in older patients with vascular risk factors (like hypertension). - However, the prominent **fever** and **leukocytosis**, coupled with recent antibiotic use, are more indicative of an infectious process like CDI than ischemia. *Presence of osmotically active, poorly absorbed solutes in the bowel lumen* - This mechanism describes **osmotic diarrhea**, which can be caused by malabsorption (e.g., lactose intolerance) or certain laxatives. - Osmotic diarrhea typically **resolves with fasting** and is not usually associated with fever, significant leukocytosis, or occult blood in the stool, which are present here.

Question 65: A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show: Hemoglobin 13.5 g/dL Leukocyte count 15,000/mm3 Platelet count 130,000/mm3 Serum Na+ 129 mEq/L Cl- 100 mEq/L K+ 4.6 mEq/L HCO3- 22 mEq/L Urea nitrogen 14 mg/dL Creatinine 1.3 mg/dL An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?

A. Urine antigen assay (Correct Answer)
B. CT Chest
C. Direct immunofluorescent antibody test
D. Stool culture
E. Polymerase chain reaction

Explanation: ***Urine antigen assay*** - This patient presents with **pneumonia symptoms** (low-grade fever, dry cough, dyspnea, bilateral infiltrates) along with **gastrointestinal symptoms** (watery diarrhea) and **hyponatremia**, after attending a hotel meeting with other sick attendees. These are classic features of **Legionnaires' disease**. - A **urine antigen assay** is a rapid and highly specific test for **Legionella pneumophila serogroup 1**, which causes the majority of Legionnaires' disease cases. *CT Chest* - A CT scan of the chest would provide more detailed imaging of the lung infiltrates but is typically used to characterize findings once pneumonia is diagnosed or to rule out other lung pathologies, not as an initial diagnostic test for the specific pathogen. - While it can reveal characteristic patterns, it doesn't identify the causative organism and is not the most appropriate *next step in diagnosis* for a presumed Legionella infection. *Direct immunofluorescent antibody test* - A **direct immunofluorescent antibody (DFA) test** is used to identify legionella in respiratory secretions. However, collecting a sufficiently good sputum sample can be difficult, especially with a **dry cough**. - Its sensitivity is lower than urine antigen testing for serogroup 1 and requires a respiratory sample, making it less convenient for initial diagnosis. *Stool culture* - While the patient has diarrhea, a **stool culture** would primarily detect typical bacterial enteric pathogens (e.g., Salmonella, Shigella, Campylobacter) and would not identify **Legionella**. - The diarrhea, in this context, is likely an extrapulmonary manifestation of Legionnaires' disease caused by Legionella, not a separate primary enteric infection. *Polymerase chain reaction* - **PCR testing** can detect Legionella DNA in respiratory samples, offering high sensitivity and specificity. - However, it is generally less rapid and widely available than the urine antigen test for initial diagnosis of Legionella pneumophila serogroup 1, which is the most common cause of Legionnaires' disease.

Question 66: A 63-year-old man comes to the physician because of shortness of breath and swollen extremities for 2 weeks. He has had excessive night sweats and a 4-kg (8.8-lb) weight loss over the last 8 weeks. He had an anterior myocardial infarction 3 years ago. He has type 2 diabetes mellitus and hypertension. He immigrated from Indonesia 4 months ago. He works in a shipyard. He has smoked one pack of cigarettes daily for 48 years. Current medications include insulin, aspirin, simvastatin, metoprolol, and ramipril. He is 160 cm (5 ft 3 in) tall and weighs 46.7 kg (103 lb); BMI is 18.2 kg/m2. His temperature is 38.0°C (100.4°F), pulse is 104/min, respirations are 20/min, and blood pressure is 135/95 mm Hg. Examination shows generalized pitting edema. There is jugular venous distention, hepatomegaly, and a paradoxical increase in jugular venous pressure on inspiration. Chest x-ray shows bilateral pleural effusion, patchy infiltrates in the right middle lobe, and pericardial thickening and calcifications. Laboratory studies show: Serum Urea nitrogen 25 mg/dL Creatinine 1.5 mg/dL Urine Blood negative Glucose negative Protein 1+ Which of the following is the most likely explanation for this patient's symptoms?

A. Viral myocarditis
B. Tuberculosis (Correct Answer)
C. Amyloidosis
D. Asbestos
E. Postmyocardial infarction syndrome

Explanation: ***Tuberculosis*** - The patient's presentation with **night sweats**, **weight loss**, **pleural effusion**, and **patchy infiltrates** is highly suggestive of tuberculosis, especially given his immigration from Indonesia (a region with high TB prevalence) and shipyard work which may expose him to crowded conditions. - **Pericardial thickening and calcifications** are strong indicators of constrictive pericarditis, a known complication of chronic tuberculosis. *Viral myocarditis* - While it can cause heart failure symptoms, it typically presents with a more acute onset of cardiac dysfunction and lacks the chronic systemic symptoms like **prolonged night sweats**, **weight loss**, and **pericardial calcification** seen here. - Patchy infiltrates on chest X-ray and pleural effusion are not primary features of isolated viral myocarditis. *Amyloidosis* - Amyloidosis can cause cardiac involvement leading to heart failure, but it usually presents with symptoms like **restrictive cardiomyopathy**, proteinuria, and organomegaly; however, **night sweats**, **significant weight loss**, and **pericardial calcification** are not typical features. - There is no mention of other common manifestations of amyloidosis such as macroglossia, periorbital purpura, or peripheral neuropathy. *Asbestos* - Asbestos exposure is associated with **pleural plaques**, **asbestosis**, and **mesothelioma**, but it does not typically cause **night sweats**, **weight loss**, or **pericardial calcifications** as described in this case. - While asbestos exposure might explain pleural findings, it doesn't account for the full clinical picture. *Postmyocardial infarction syndrome* - **Dressler's syndrome** (post-MI syndrome) can cause pericarditis and pleuritis after a myocardial infarction, but it typically occurs weeks to months after the event, and this patient's MI was 3 years ago. - Furthermore, Dressler's syndrome does not cause the **chronic night sweats**, **significant weight loss**, or **pericardial calcifications** observed in this patient.

Question 67: A 24-year-old woman is brought to the emergency department by friends because of an episode of jerking movements of the whole body that lasted for one minute. She reports a 2-week history of fever, headache, and altered sensorium. Her fever ranges from 38.3°C (101.0°F) to 38.9°C (102.0°F). Her past medical history is significant for toothache and multiple dental caries. The patient denies any history of smoking or alcohol or drug use. She is not currently sexually active. Her vital signs include: blood pressure 110/74 mm Hg, pulse 124/min, respiratory rate 14/min, temperature 38.9°C (102.0°F). On physical examination, the patient is confused and disoriented. She is moving her right side more than her left. A noncontrast CT scan of the head reveals a ring-enhancing lesion in the left frontal lobe consistent with a cerebral abscess. The abscess is evacuated and sent for culture studies. Which of the following microorganisms did the culture most likely grow?

A. Escherichia coli
B. Actinomyces israelii
C. Streptococcus viridans (Correct Answer)
D. Staphylococcus aureus
E. Pseudomonas aeruginosa

Explanation: ***Streptococcus viridans*** - The patient's history of **dental caries** and **toothache** points to an oral source of infection, and *S. viridans* is a common organism in the oral flora that can cause **cerebral abscesses**, especially after dental procedures or poor dental hygiene. - The presence of a **ring-enhancing lesion** on CT scan, along with fever, headache, altered sensorium, and seizures, is classic for a **cerebral abscess**. *Escherichia coli* - *E. coli* is a common cause of **urinary tract infections** and **intra-abdominal infections**, but it is an uncommon cause of cerebral abscess in immunocompetent individuals. - While it can cause meningitis in neonates and immunocompromised patients, it's not typically linked to dental sources or cerebral abscesses in this demographic. *Actinomyces israelii* - *Actinomyces israelii* can cause **abscesses** (actinomycosis), often in the cervicofacial region, and it is associated with poor oral hygiene. - However, it typically leads to a more **indolent, slowly progressive infection** with characteristic draining sinuses, which is less consistent with the acute presentation and single cerebral abscess described. *Staphylococcus aureus* - *Staphylococcus aureus* is a significant cause of **cerebral abscesses**, particularly in patients with **bacteremia**, trauma, or neurosurgical procedures. - While possible, the strong association with dental caries makes *Streptococcus viridans* a more specific and likely pathogen in this scenario. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* typically causes cerebral abscesses in **immunocompromised patients**, those with **nosocomial infections**, or after neurosurgical procedures. - The patient's history does not suggest these risk factors, making *Pseudomonas* less likely.

Question 68: An investigator is studying the effect of different cytokines on the growth and differentiation of B cells. The investigator isolates a population of B cells from the germinal center of a lymph node. After exposure to a particular cytokine, these B cells begin to produce an antibody that prevents attachment of pathogens to mucous membranes but does not fix complement. Which of the following cytokines is most likely responsible for the observed changes in B-cell function?

A. Interleukin-5 (Correct Answer)
B. Interleukin-6
C. Interleukin-8
D. Interleukin-2
E. Interleukin-4

Explanation: ***Interleukin-5*** - The antibody described (prevents pathogen attachment to mucous membranes, does not fix complement) is characteristic of **IgA**. - **IL-5** plays a crucial role in promoting **IgA secretion** by differentiated B cells and supports B cell growth and differentiation in mucosal immunity. - IL-5 works synergistically with **TGF-β** (the primary cytokine for IgA class switching) to enhance IgA production, particularly in mucosal-associated lymphoid tissue. - Among the options provided, **IL-5 has the strongest association with IgA production**. *Interleukin-4* - **IL-4** is the primary cytokine driving class switching to **IgE** (and IgG4), not IgA. - IL-4 is central to **allergic responses** and type 2 immunity, promoting B cells to produce IgE antibodies against allergens and parasites. - It does not play a significant role in IgA production or mucosal immunity. *Interleukin-6* - **IL-6** is a pleiotropic cytokine involved in acute phase reactions, inflammation, and promoting B cell **differentiation into plasma cells**. - While it supports general B cell maturation and antibody secretion, it is not specifically associated with **IgA production** or class switching. *Interleukin-8* - **IL-8** (CXCL8) is a **chemokine** that primarily recruits and activates neutrophils during inflammation. - It has no direct role in B cell class switching or antibody production. *Interleukin-2* - **IL-2** is essential for T cell proliferation and differentiation, enhancing **cell-mediated immunity**. - While it can indirectly affect B cell responses through T cell help, it is not directly responsible for promoting **IgA production** or class switching.

Question 69: A father brings his 3-year-old son to the pediatrician because he is concerned about his health. He states that throughout his son's life he has had recurrent infections despite proper treatment and hygiene. Upon reviewing the patient's chart, the pediatrician notices that the child has been infected multiple times with S. aureus, Aspergillus, and E. coli. Which of the following would confirm the most likely cause of this patient's symptoms?

A. Increased IgM, Decreased IgG, IgA, and IgE
B. Negative nitroblue-tetrazolium test (Correct Answer)
C. Positive nitroblue-tetrazolium test
D. Normal dihydrorhodamine (DHR) flow cytometry test
E. Increased IgE and IgA, Decreased IgM

Explanation: ***Negative nitroblue-tetrazolium test*** - A **negative nitroblue-tetrazolium (NBT) test** indicates an inability of phagocytes to produce a respiratory burst, which is characteristic of **Chronic Granulomatous Disease (CGD)**. - CGD patients suffer from recurrent infections with catalase-positive organisms such as *Staphylococcus aureus*, *Aspergillus*, and *E. coli*, consistent with the patient's history. *Increased IgM, Decreased IgG, IgA, and IgE* - This pattern of immunoglobulin levels is characteristic of **X-linked hyper-IgM syndrome**, where there is a defect in CD40L on T cells. - While it also causes recurrent infections, the typical pathogens differ from those stated in the question, often including *Pneumocystis jirovecii*. *Positive nitroblue-tetrazolium test* - A **positive NBT test** indicates that phagocytes are capable of producing a respiratory burst and forming superoxide, thus ruling out CGD. - This result would be expected in a healthy individual or someone with an immunodeficiency not affecting the phagocytic oxidative burst. *Normal dihydrorhodamine (DHR) flow cytometry test* - A **normal DHR flow cytometry test** indicates that neutrophils can produce reactive oxygen species (ROS) effectively, meaning the respiratory burst is intact. - This result would rule out CGD, as CGD patients have an abnormal (decreased) DHR test. *Increased IgE and IgA, Decreased IgM* - This specific pattern of immunoglobulin abnormalities is not typically associated with a single, well-defined primary immunodeficiency that would present with the described infections. - **Hyper-IgE syndrome (Job's syndrome)**, for example, features very high IgE levels but usually a normal IgM.

Question 70: A 3-year-old boy presents to the pediatrics clinic for follow-up. He has a history of severe pyogenic infections since birth. Further workup revealed a condition caused by a defect in CD40 ligand expressed on helper T cells. This congenital immunodeficiency has resulted in an inability to class switch and a poor specific antibody response to immunizations. Which of the following best characterizes this patient's immunoglobulin profile?

A. Increased IgM; decreased IgG, IgA, and IgE (Correct Answer)
B. Increased IgE and IgA; and decreased IgM
C. Increased IgE
D. Decreased IgA
E. Decreased Interferon gamma

Explanation: ***Increased IgM; decreased IgG, IgA, and IgE*** - The described condition is **Hyper-IgM Syndrome** due to a defect in **CD40 ligand**, which prevents B cells from class-switching. - This results in the continued production of **IgM** (which does not require CD40L for its synthesis) and *decreased levels of all other immunoglobulin isotypes* (IgG, IgA, IgE). *Increased IgE and IgA; and decreased IgM* - This profile is not characteristic of Hyper-IgM syndrome; it might be seen in other immunodeficiencies but not one caused by a CD40L defect. - A decrease in IgM production would indicate a defect in early B-cell development or IgM synthesis, which is contrary to the features of Hyper-IgM syndrome where IgM is typically high. *Increased IgE* - While increased IgE can be a feature of certain immunodeficiencies (e.g., Job's syndrome/Hyper-IgE syndrome), it is not the primary or sole immunoglobulin abnormality in **Hyper-IgM Syndrome**. - In Hyper-IgM syndrome, IgE is typically decreased due to the inability to class switch to IgE. *Decreased IgA* - This is true in Hyper-IgM syndrome, but it's only one part of the immunoglobulin profile and not the most comprehensive answer. - Isolated decreased IgA (Selective IgA Deficiency) is a separate primary immunodeficiency with different clinical manifestations and underlying molecular defects. *Decreased Interferon gamma* - Interferon gamma is a cytokine produced by T cells (Th1 cells) and NK cells, important for antiviral and anti-intracellular bacterial immunity. - While T-cell function can be impaired in immunodeficiencies, a defect in CD40L primarily affects B-cell class switching and antibody production, not directly interferon gamma levels.

Question 71: A 60-year-old man who recently immigrated from South America schedules an appointment with a physician to complete his pre-employment health clearance form. According to company policy, a skin test for tuberculosis must be administered to all new employees. Thus, he received an intradermal injection of purified protein derivative (PPD) on his left forearm. After 48 hours, a 14-mm oval induration is noticed. The type of cells most likely present and responsible for the indurated area will have which of the following characteristic features?

A. They are rich in myeloperoxidase enzyme.
B. They need thymus for their maturation. (Correct Answer)
C. They play an important part in allergic reactions.
D. They have multiple-lobed nucleus.
E. Their half-life is 24–48 hours.

Explanation: ***They need thymus for their maturation.*** - The induration in a **PPD test** is mediated by **T-helper cells (CD4+)**, which are a type of **T lymphocyte**. - **T lymphocytes** mature in the **thymus**, where they undergo positive and negative selection. *They are rich in myeloperoxidase enzyme.* - Cells rich in **myeloperoxidase enzyme** are primarily **neutrophils**, which are involved in acute inflammation and bacterial killing. - The PPD induration is a **Type IV hypersensitivity reaction**, characterized by a delayed, cell-mediated immune response, not an acute neutrophil-driven one. *They play an important part in allergic reactions.* - Cells primarily involved in allergic reactions (**Type I hypersensitivity**) are **mast cells** and **basophils**, which release mediators like histamine. - The PPD test is a **Type IV, delayed-type hypersensitivity**, involving T-cells and macrophages, not allergic reactions. *They have multiple-lobed nucleus.* - Cells with a **multiple-lobed nucleus** are characteristic of **neutrophils**, as well as other granulocytes like eosinophils and basophils. - **T lymphocytes** typically have a large, round nucleus that fills most of the cytoplasm. *Their half-life is 24–48 hours.* - Cells with a short half-life of 24-48 hours are characteristic of some granulocytes, like **neutrophils**, which have a relatively short lifespan. - **Memory T lymphocytes**, which are crucial for the PPD response, can persist for many years, providing long-term immunity.

Question 72: A 42-year-old man presents with an intermittent low-to-high grade fever, night sweats, weight loss, fatigue, and exercise intolerance. The symptoms have been present for the last 6 months. The patient is a software developer. He smokes one-half pack of cigarettes daily and drinks alcohol occasionally. He denies intravenous drug use. There is no history of cardiovascular, respiratory, or gastrointestinal diseases or malignancies. There is no family history of cancer or cardiovascular diseases. The only condition he reports is a urinary bladder polyp, which was diagnosed and removed endoscopically almost 8 months ago. The patient does not currently take any medications. His blood pressure is 100/80 mm Hg, heart rate is 107/min, respiratory rate is 19/min, and temperature is 38.1°C (100.6°F). The patient is ill-looking and pale. There are several petechial conjunctival hemorrhages and macular lesions on both palms. The cardiac examination reveals heart enlargement to the left side and a holosystolic murmur best heard at the apex of the heart. There is also symmetric edema in both legs up to the knees. Which of the following organisms is most likely to be cultured from the patient’s blood?

A. Pseudomonas aeruginosa
B. Candida albicans
C. Streptococcus viridans (Correct Answer)
D. Staphylococcus aureus
E. Enterococcus faecalis

Explanation: ***Streptococcus viridans*** - The patient's symptoms (fever, night sweats, weight loss, fatigue, new-onset **holosystolic murmur**, **petechial hemorrhages**, **macular lesions on palms**) are highly suggestive of **subacute infective endocarditis**. - **Viridans streptococci** are the most common cause of **subacute bacterial endocarditis** overall, accounting for 50-60% of cases on previously damaged heart valves or in individuals with predisposing conditions. - While the patient had a bladder polyp removed **8 months ago**, the remote timing (well beyond typical endocarditis prophylaxis windows) and the **6-month duration of symptoms** make viridans the most likely pathogen statistically. *Pseudomonas aeruginosa* - This organism primarily causes endocarditis in **intravenous drug users** or in patients with **prosthetic valves**, which are not present in this case. - The patient denies IVDU, making this organism less likely. *Candida albicans* - **Fungal endocarditis** due to *Candida* is rare and typically occurs in immunocompromised individuals, those with prosthetic valves, or long-term central venous catheter users. - There is no indication of immunocompromise or prosthetic valves in this patient. *Staphylococcus aureus* - **Staphylococcus aureus** causes **acute infective endocarditis**, often presenting with rapid onset and severe symptoms over days to weeks, frequently involving healthy valves and **intravenous drug users**. - The patient's **6-month history** of symptoms points to a subacute, rather than an acute, process, making S. aureus less likely. *Enterococcus faecalis* - **Enterococcus faecalis** is indeed associated with endocarditis following **genitourinary procedures** (including cystoscopy with polypectomy). - However, the **8-month interval** between the GU procedure and symptom onset is quite remote; most procedure-related endocarditis develops within weeks to a few months. - Additionally, while enterococcus causes 5-15% of endocarditis cases, **viridans streptococci remain the most common cause of subacute endocarditis overall**, making it the statistically more likely pathogen in this clinical presentation.

Question 73: Two weeks after undergoing an allogeneic skin graft procedure for extensive full-thickness burns involving his left leg, a 41-year-old man develops redness and swelling over the graft site. He has not had any fevers or chills. His temperature is 36°C (96.8°F). Physical examination of the left lower leg shows well-demarcated erythema and edema around the skin graft site. The graft site is minimally tender and there is no exudate. Which of the following is the most likely underlying mechanism of this patient’s skin condition?

A. Th1-induced macrophage activation (Correct Answer)
B. Staphylococci-induced neutrophil activation
C. Immune complex-mediated complement activation
D. Opsonization-induced cell destruction
E. Antibody-mediated complement activation

Explanation: - **Th1-induced macrophage activation** is the mechanism behind **acute allograft rejection**, which typically occurs within days to weeks post-transplant and involves **T-lymphocytes** recognizing donor antigens. - The symptoms of localized redness, swelling, and edema around the graft site, without systemic signs of infection or exudate, are highly consistent with a **cellular immune response** directed against the foreign tissue. *Staphylococci-induced neutrophil activation* - **Bacterial infection** usually presents with signs of inflammation like warmth, severe pain, purulent discharge (exudate), and potentially fever and chills. - The patient's lack of fever, chills, and exudate, coupled with only minimal tenderness, makes a **bacterial infection less likely**. *Immune complex-mediated complement activation* - This mechanism is characteristic of **Type III hypersensitivity reactions**, such as **serum sickness** or **Arthus reaction**. - These conditions typically present with vasculitis, glomerulonephritis, or arthritis, which are not described in the patient's localized skin graft rejection. *Opsonization-induced cell destruction* - **Opsonization** primarily involves tagging cells for destruction by **phagocytes** and is mediated by antibodies (IgG) or complement proteins (C3b). - While it can be part of an immune response, it is generally associated with **autoimmune hemolytic anemia** or the removal of infectious agents, not the primary mechanism of acute cellular graft rejection. *Antibody-mediated complement activation* - **Antibody-mediated complement activation** is characteristic of **hyperacute rejection**, which occurs **minutes to hours** after transplantation due to pre-formed antibodies. - It can also be involved in **acute humoral rejection**, but the two-week timeframe and the localized, inflammatory nature of the symptoms without vascular compromise suggest a **cell-mediated response** as the primary mechanism.

Question 74: A 38-year-old woman comes to the physician because of a 4-day history of swelling and pain in her left knee. She has had similar episodes of swollen joints over the past 3 weeks. Two months ago, she had a rash on her upper back that subsided after a few days. She lives in Pennsylvania and works as a forest ranger. Her temperature is 37.8°C (100°F). Physical examination shows a tender and warm left knee. Arthrocentesis of the knee joint yields cloudy fluid with a leukocyte count of 65,000/mm3 and 80% neutrophils. A Gram stain of synovial fluid does not show any organisms. Which of the following is the most likely cause of this patient's condition?

A. Production of autoantibodies against Fc portion of IgG
B. Wearing down of articular cartilage
C. Postinfectious activation of innate lymphoid cells of the gut
D. Infection with spiral-shaped bacteria (Correct Answer)
E. Infection with round bacteria in clusters

Explanation: ***Infection with spiral-shaped bacteria*** - This clinical presentation, including multifocal arthritis following a rash, a mild fever, and a history of working as a forest ranger in Pennsylvania (an **endemic area**), is highly suggestive of **Lyme arthritis**. - Lyme disease is caused by the **spirochete** *Borrelia burgdorferi*, a **spiral-shaped bacterium**, transmitted by tick bites. *Production of autoantibodies against Fc portion of IgG* - This describes the presence of **rheumatoid factor (RF)**, which is characteristic of **rheumatoid arthritis (RA)**. - RA typically presents with chronic, symmetrical polyarthritis, often involving the small joints, and does not commonly begin with an acute rash or affect forest rangers specifically. *Wearing down of articular cartilage* - This mechanism is characteristic of **osteoarthritis (OA)**, which is a degenerative joint disease. - OA typically affects older individuals, is not associated with acute inflammatory episodes, fevers, or the presence of a rash, and the synovial fluid analysis would show fewer inflammatory cells. *Postinfectious activation of innate lymphoid cells of the gut* - This mechanism is associated with **reactive arthritis** or spondyloarthropathies, which are typically triggered by gastrointestinal or genitourinary infections. - While reactive arthritis can cause acute arthritis, the prodromal rash, geographic location, and specific sequence of symptoms point away from a gut-derived trigger. *Infection with round bacteria in clusters* - **Round bacteria in clusters** (Gram-positive cocci in clusters) typically refers to **Staphylococcus aureus**, a common cause of **septic arthritis**. - While septic arthritis can cause acute, painful, swollen joints with high synovial fluid leukocyte counts, the history of a preceding rash and multifocal, migratory joint involvement makes Lyme arthritis more likely; also, a Gram stain for *Staphylococcus* would typically be positive.

Question 75: A 4-month-old boy is brought to the physician by his parents because of fever for the past 3 days. They also state that he has been less active and has been refusing to eat. The patient has had two episodes of bilateral otitis media since birth. He was born at term and had severe respiratory distress and sepsis shortly after birth that was treated with antibiotics. Umbilical cord separation occurred at the age of 33 days. The patient appears pale. Temperature is 38.5°C (101.3°F), pulse is 170/min, and blood pressure is 60/40 mm Hg. He is at the 25th percentile for height and 15th percentile for weight. Examination shows a capillary refill time of 4 seconds. Oral examination shows white mucosal patches that bleed when they are scraped off. There is bilateral mucoid, nonpurulent ear discharge. Several scaly erythematous skin lesions are seen on the chest. Laboratory studies show a leukocyte count of 38,700/mm3 with 90% neutrophils and a platelet count of 200,000/mm3. Which of the following is the most likely underlying cause of this patient's symptoms?

A. Defective NADPH oxidase
B. Defective IL-2R gamma chain
C. WAS gene mutation
D. Defective beta-2 integrin (Correct Answer)
E. Defective lysosomal trafficking regulator gene

Explanation: ***Defective beta-2 integrin*** - This condition, known as **Leukocyte Adhesion Deficiency Type 1 (LAD-1)**, is characterized by **defective neutrophil adhesion** due to mutations in the *CD18* gene, which encodes the beta-2 integrin subunit. This leads to impaired leukocyte extravasation into tissues. - Clinical features include recurrent bacterial infections (like **otitis media**), severe periodontal disease, **impaired wound healing** with delayed umbilical cord separation (>30 days), and **marked peripheral leukocytosis with neutrophilia** (as seen in this patient) because neutrophils cannot exit the bloodstream. *Defective NADPH oxidase* - This describes **Chronic Granulomatous Disease (CGD)**, where phagocytes cannot produce reactive oxygen species. Patients suffer from recurrent bacterial and fungal infections, often by catalase-positive organisms, and **granuloma formation**. - While patients with CGD do have recurrent infections, **delayed umbilical cord separation** and the specific presentation of neutrophilic leukocytosis due to inability to extravasate are not characteristic features of CGD. *Defective IL-2R gamma chain* - Mutations in the **IL-2R gamma chain** lead to **X-linked Severe Combined Immunodeficiency (SCID)**, characterized by a **lack of T cells and NK cells**, making infants highly susceptible to severe opportunistic infections. - This patient presents with neutrophilic leukocytosis and features more indicative of a neutrophil defect rather than a primary T-cell deficiency; SCID typically presents with severe bacterial, viral, fungal, and parasitic infections early in life and often involves failure to thrive. *WAS gene mutation* - A mutation in the *WAS* (Wiskott-Aldrich Syndrome) gene causes **Wiskott-Aldrich Syndrome**, characterized by the triad of **thrombocytopenia** (small platelets), **eczema**, and **recurrent infections**. - This patient has a normal platelet count (200,000/mm3), which rules out Wiskott-Aldrich Syndrome. *Defective lysosomal trafficking regulator gene* - This defect is associated with **Chediak-Higashi syndrome**, which involves impaired phagolysosome formation in phagocytes. Clinical features include **partial albinism**, recurrent pyogenic infections, peripheral neuropathy, and large granules in leukocytes. - Although recurrent infections are present, the patient's symptoms do not include albinism or an abnormal morphology of neutrophils with giant lysosomes. Additionally, **delayed umbilical cord separation** is not a hallmark of Chediak-Higashi syndrome.

Question 76: Fluorescein is an artificial dark orange colored organic molecule used in the diagnosis of corneal ulcers and herpetic corneal infections. It is observed that, in experimental animals, the fluorescein binds to receptors on certain B cells, but it does not stimulate them to produce fluorescein specific antibodies unless it is first attached to a larger molecule such as albumin. Which of the following terms best describes fluorescein?

A. Tolerogen
B. Hapten (Correct Answer)
C. Immunogen
D. Carrier
E. Adjuvant

Explanation: ***Hapten*** - A **hapten** is a small molecule that can bind to B cell receptors and be recognized by antibodies but **cannot by itself induce an immune response**. - For a hapten to become immunogenic (stimulate antibody production), it must be **covalently linked to a larger carrier molecule**, such as a protein like albumin, which provides the necessary T cell help. *Tolerogen* - A **tolerogen** is an antigen that, under specific circumstances, **induces immune unresponsiveness (tolerance)** rather than an immune response. - This typically involves rendering lymphocytes anergic or causing their deletion, preventing future responses to that specific antigen. *Immunogen* - An **immunogen** is any substance that is capable of **eliciting a humoral or cell-mediated immune response** on its own. - These molecules are typically large and complex enough to be recognized by the immune system and drive antibody production or T cell activation without requiring a carrier molecule. *Carrier* - A **carrier protein** is a large molecule to which a hapten is conjugated to make it **immunogenic**. - In this scenario, albumin serves as the carrier molecule, but fluorescein itself is not the carrier; it is the molecule requiring the carrier for immunogenicity. *Adjuvant* - An **adjuvant** is a substance that **enhances the immune response to an antigen** when administered together, but it is not itself specifically targeted by the immune response. - Adjuvants work by mechanisms such as prolonging antigen presentation, enhancing co-stimulatory signals, or inducing local inflammation, rather than being the immunogenic component itself.

Question 77: A 24-year-old college student consumed a container of canned vegetables for dinner. Fourteen hours later, he presents to the E.R. complaining of difficulty swallowing and double-vision. The bacterium leading to these symptoms is:

A. Rod-shaped (Correct Answer)
B. An obligate aerobe
C. Gram-negative
D. Non-spore forming
E. Cocci-shaped

Explanation: ***Rod-shaped*** - The symptoms of **difficulty swallowing (dysphagia)** and **double vision (diplopia)**, following consumption of canned food, are highly indicative of **botulism**, caused by *Clostridium botulinum*. - *Clostridium botulinum* is a **rod-shaped (bacillus)** bacterium, which is a key morphological characteristic. *An obligate aerobe* - *Clostridium botulinum* is actually an **obligate anaerobe**, meaning it thrives in environments without oxygen, such as sealed cans. - An obligate aerobe requires oxygen for growth; therefore, this statement is incorrect. *Gram-negative* - *Clostridium botulinum* is a **Gram-positive bacterium**, specifically a Gram-positive rod. - Gram-negative bacteria have a different cell wall structure and stain pink/red, unlike the purple of Gram-positive bacteria. *Non-spore forming* - *Clostridium botulinum* is a **spore-forming bacterium**, and its spores are famously heat-resistant, allowing them to survive inadequate canning processes. - The formation of spores is crucial for its survival and persistence in various environments. *Cocci-shaped* - **Cocci-shaped** bacteria are spherical, but *Clostridium botulinum* is explicitly **rod-shaped (bacillus)**. - This morphological description does not match the causative agent of botulism.

Question 78: A 25-year-old professional surfer presents to the emergency room with leg pain and a headache. He recently returned from a surf competition in Hawaii and has been feeling unwell for several days. He regularly smokes marijuana and drinks 6-7 beers during the weekend. He is otherwise healthy and does not take any medications. His temperature is 102.2°F (39°C), blood pressure is 121/78 mmHg, pulse is 120/min, and respirations are 18/min saturating 99% on room air. He is sitting in a dim room as the lights bother his eyes and you notice scleral icterus on physical exam. Cardiopulmonary exam is unremarkable. Which of the following findings would most likely be seen in this patient?

A. Epithelial cells covered with gram-variable rods
B. Treponemes on dark-field microscopy
C. Granulocytes with morulae in the cytoplasm
D. Question mark-shaped bacteria on dark-field microscopy (Correct Answer)
E. Monocytes with morulae in the cytoplasm

Explanation: ***Question mark-shaped bacteria on dark-field microscopy*** The patient's symptoms (fever, headache, leg pain, photophobia, and especially **scleral icterus** after returning from Hawaii) are highly suggestive of **leptospirosis**. **Leptospira interrogans** is a spirochete characterized by its **question mark-shape** and can be visualized using **dark-field microscopy** in early stages of infection from blood or CSF, and later in urine. *Epithelial cells covered with gram-variable rods* This description is characteristic of **bacterial vaginosis**, where epithelial cells are covered by **Gardnerella vaginalis** and other bacteria, forming "clue cells." This condition is a genital infection and does not align with the systemic symptoms and exposures described in the patient. *Treponemes on dark-field microscopy* **Treponema pallidum** is the causative agent of **syphilis**, which is also a spirochete and can be identified by dark-field microscopy from chancres. While this is a spirochetal infection, the clinical presentation of fever, leg pain, severe headache, photophobia, and jaundice (scleral icterus) following exposure to tropical waters is characteristic of leptospirosis, not early syphilis. *Granulocytes with morulae in the cytoplasm* **Morulae** (intracellular mulberry-shaped aggregates of bacteria) in the cytoplasm of **granulocytes** are characteristic of **anaplasmosis**, caused by Anaplasma phagocytophilum. While anaplasmosis can cause fever and headache, it is typically a tick-borne illness and does not explain the prominent **icterus** or the Hawaii exposure in this context. *Monocytes with morulae in the cytoplasm* **Morulae** within the cytoplasm of **monocytes** are characteristic of **ehrlichiosis**, caused by Ehrlichia chaffeensis. Like anaplasmosis, ehrlichiosis is a tick-borne disease and does not fit the epidemiological context (surfing in Hawaii) or the specific clinical picture (notably **icterus**) as well as leptospirosis.

Question 79: A 17-year-old girl is brought to the emergency department by her father with fever, chills, and a body rash. Her father reports that 3 days ago, his daughter underwent surgery for a deviated nasal septum. Since then, she has been "sleepy" and in moderate pain. When the patient’s father came home from work today, he found the patient on the couch, shivering and complaining of muscle aches. He also noticed a rash all over her body. The patient says she feels “hot and cold” and also complains of lightheadedness. The patient has no other past medical history. She has been taking oxycodone for the post-surgical pain. She denies any recent travel. The father reports the patient’s brother had a minor “cold” last week. The patient’s mother has major depressive disorder. The patient denies tobacco or illicit drug use. She says she has tried beer before at parties. Her temperature is 103.2°F (39.6°C), blood pressure is 84/53, pulse is 115/min, respirations are 12/min, and oxygen saturation is 99% on room air. The patient is awake and oriented but slow to respond. There is no focal weakness or nuchal rigidity. Physical examination reveals nasal packing in both nostrils, tachycardia, and a diffuse, pink, macular rash that is also present on the palms and soles. Initial labs show a neutrophil-dominant elevation in leukocytes, a creatinine of 2.1 mg/dL, an aspartate aminotransferase of 82 U/L, and an alanine aminotransferase of 89 U/L. Which of the following is the most likely cause of the patient’s symptoms?

A. Polyclonal T-cell activation (Correct Answer)
B. Bacterial lysis
C. Mast cell degranulation
D. Opioid receptor stimulation
E. Circulating endotoxin

Explanation: ***Polyclonal T-cell activation*** - The patient's symptoms (fever, rash, hypotension, multi-organ dysfunction, recent surgery with nasal packing) are highly suggestive of **toxic shock syndrome (TSS)**. - TSS is caused by **superantigens**, primarily from *Staphylococcus aureus*, which directly cross-link MHC class II molecules and T-cell receptors, leading to massive **polyclonal T-cell activation** and cytokine storm. *Bacterial lysis* - While bacteria are the underlying cause of TSS, **bacterial lysis** itself is not the direct mechanism for the systemic symptoms like rash and shock. - Bacterial lysis is more relevant to the release of **endotoxins** (from Gram-negative bacteria) or intracellular components, but TSS is typically associated with Gram-positive *S. aureus* exotoxins. *Mast cell degranulation* - **Mast cell degranulation** is primarily involved in **allergic reactions** and anaphylaxis, leading to histamine release and localized or systemic hypersensitivity. - The clinical presentation of TSS, with its prominent rash, fever, and multi-organ involvement, is distinct from an anaphylactic reaction. *Opioid receptor stimulation* - **Opioid receptor stimulation** by analgesics like oxycodone can cause sedation, respiratory depression, and constipation; however, it does not explain the fever, rash, hypotension, or signs of multi-organ injury (elevated creatinine and liver enzymes). - The patient's presentation is a systemic inflammatory response, not an opioid side effect or overdose. *Circulating endotoxin* - **Circulating endotoxin** (lipopolysaccharide from Gram-negative bacteria) can cause **septic shock** with fever, hypotension, and organ dysfunction, but the rash in endotoxic shock is typically purpuric or petechial, not a diffuse macular rash involving palms and soles. - The classic diffuse macular rash on palms and soles is characteristic of **toxic shock syndrome**, primarily mediated by *Staphylococcus aureus* exotoxins (superantigens), not endotoxins.

Question 80: A 45-year-old man comes to the emergency department because of chills and numerous skin lesions for 1 week. He has also had watery diarrhea, nausea, and abdominal pain for the past 2 weeks. The skin lesions are nonpruritic and painless. He was diagnosed with HIV infection approximately 20 years ago. He has not taken any medications for over 5 years. He sleeps in homeless shelters and parks. Vital signs are within normal limits. Examination shows several bright red, friable nodules on his face, trunk, extremities. The liver is palpated 3 cm below the right costal margin. His CD4+ T-lymphocyte count is 180/mm3 (N ≥ 500). A rapid plasma reagin test is negative. Abdominal ultrasonography shows hepatomegaly and a single intrahepatic 1.0 x 1.2-cm hypodense lesion. Biopsy of a skin lesion shows vascular proliferation and abundant neutrophils. Which of the following is the most likely causal organism?

A. Bartonella henselae (Correct Answer)
B. Treponema pallidum
C. HHV-8 virus
D. Mycobacterium avium
E. Candida albicans

Explanation: ***Bartonella henselae*** - The patient's presentation with **bright red, friable nodules** (consistent with **bacillary angiomatosis**) in an HIV-positive individual with a low **CD4+ count** strongly suggests infection with *Bartonella henselae*. - **Hepatomegaly** and **intrahepatic lesions** further support disseminated bartonellosis, and skin biopsy showing vascular proliferation with **abundant neutrophils** is characteristic. *Treponema pallidum* - While *Treponema pallidum* (syphilis) can cause various skin lesions, the **rapid plasma reagin (RPR) test** was negative, making syphilis highly unlikely. - Syphilitic lesions typically do not present as brightly friable nodules with prominent vascular proliferation and neutrophils characteristic of bacillary angiomatosis. *HHV-8 virus* - **HHV-8** is the causative agent of **Kaposi sarcoma**, which also presents with vascular lesions. However, Kaposi sarcoma lesions are typically **violaceous plaques or nodules** and histologically show spindle cells and extravasated red blood cells, not the prominent neutrophils seen here. - The patient's clinical presentation, particularly the friable nature and specific histology, steers away from Kaposi sarcoma. *Mycobacterium avium* - *Mycobacterium avium* complex (MAC) can cause disseminated disease in HIV patients with low CD4 counts, often presenting with fever, weight loss, and gastrointestinal symptoms. - However, MAC infection rarely causes specific nonpruritic, bright red, friable skin nodules like those described, and hepatic lesions would typically be granulomatous, not necessarily angiomatous. *Candida albicans* - While *Candida albicans* can cause various infections in immunocompromised individuals, including esophagitis and mucocutaneous candidiasis, it does not typically present with these specific bright red, friable vascular skin nodules. - Disseminated candidiasis would more likely involve fungemia and widespread organ involvement, often with more subtle or different skin manifestations (e.g., maculopapular rash).

Question 81: Two days after hospital admission and surgical treatment for a cut on his right thigh from a sickle, a 35-year-old man has fever, chills, and intense pain. The wound is swollen. He had a similar injury 4 months ago that resolved following treatment with bacitracin ointment and daily dressings. He works on a farm on the outskirts of the city. He appears anxious. His temperature is 38.5°C (101.3°F), pulse is 103/min, and blood pressure is 114/76 mm Hg. Examination shows a 6-cm edematous deep, foul-smelling wound on the medial surface of the right thigh. The skin over the thigh appears darker than the skin on the lower leg. There are multiple blisters around the wound. Light palpation around the wound causes severe pain; crepitus is present. Which of the following is the most likely causal organism?

A. Staphylococcus aureus
B. Clostridium perfringens (Correct Answer)
C. Pseudomonas aeruginosa
D. Rhizopus oryzae
E. Pasteurella multocida

Explanation: ***Clostridium perfringens*** - The rapid onset of severe pain, **foul-smelling discharge**, **crepitus** (due to gas production), **edema**, and **blisters** in a deep wound are classic signs of **gas gangrene**, most commonly caused by *Clostridium perfringens*. - This organism is a **spore-forming anaerobe** commonly found in soil, consistent with the patient's farm work and the nature of the injury (sickle cut leading to a deep wound). *Staphylococcus aureus* - While *Staphylococcus aureus* can cause wound infections, it typically presents with **abscess formation**, **purulent drainage**, and **erythema** without crepitus or the rapid, severe tissue destruction seen here. - It is not associated with the **foul-smelling discharge** or **gas production** characteristic of gas gangrene. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* infections often occur in **burns**, puncture wounds (especially through shoes), and in immunocompromised patients, producing a **grape-like odor** and a **blue-green pigment**. - It does not typically cause gas gangrene with crepitus or the rapid tissue necrosis described in the patient. *Rhizopus oryzae* - *Rhizopus oryzae* is a **fungus** that causes **mucormycosis**, primarily affecting immunocompromised individuals, usually presenting as rhinocerebral, pulmonary, or cutaneous infections. - It is not a bacterial cause of acute, rapidly spreading wound infections with gas production like the one described. *Pasteurella multocida* - *Pasteurella multocida* is typically associated with **animal bites** or scratches, causing rapid-onset cellulitis and local infection. - While it can cause soft tissue infection, it does not produce gas or the profound tissue destruction and foul odor seen in gas gangrene.

Question 82: A 10-year-old boy from Sri Lanka suffers from an autosomal dominant condition, the hallmark of which is hyperimmunoglobulinemia E and eosinophilia. He suffers from recurrent infections and takes antibiotic chemoprophylaxis. A STAT3 mutation analysis has been performed to confirm the diagnosis of Job syndrome. I. Eosinophilia II. Eczema III. Hay fever IV. Atopic dermatitis V. Recurrent skin and lung infections VI. Bronchial asthma What combination of symptoms above is characteristic of this condition?

A. I, II, III
B. I, II, IV, V
C. I, III, IV
D. I, II, V (Correct Answer)
E. IV, V, VI

Explanation: ***I, II, V*** - **Job syndrome** (Hyper-IgE syndrome) is characterized by a triad of **recurrent infections** (recurrent skin and lung infections), **severe eczema**, and **eosinophilia**. - The patient's presentation with hyperimmunoglobulinemia E, eosinophilia, and recurrent infections, coupled with a confirmed **STAT3 mutation**, directly aligns with the key features of this syndrome. *I, II, III* - While **eosinophilia** and **eczema** are characteristic of Job syndrome, **hay fever** (allergic rhinitis) is not a primary diagnostic criterion, although allergic manifestations can occur. - The core clinical presentation focuses on immune dysregulation leading to infections and severe skin involvement, which are more specific than hay fever. *I, II, IV, V* - This option includes **atopic dermatitis** which is essentially synonymous with **eczema**, leading to redundancy. - The combination should focus on distinct major clinical features rather than overlapping terms. *I, III, IV* - This option includes **hay fever** and **atopic dermatitis**, neither of which are as central to the diagnostic criteria as recurrent infections. - The most critical elements for diagnosis are the immune dysfunction leading to severe infections and the hallmark skin condition. *IV, V, VI* - This option lists **atopic dermatitis**, **recurrent skin and lung infections**, and **bronchial asthma**. While these can be seen in Job syndrome, it omits **eosinophilia**, which is a defining laboratory finding. - **Bronchial asthma** is also not a primary diagnostic feature, and the absence of eosinophilia makes this option less accurate.

Question 83: A 33-year-old woman presents to the urgent care center with 4 days of abdominal pain and increasingly frequent bloody diarrhea. She states that she is currently having 6 episodes of moderate volume diarrhea per day with streaks of blood mixed in. Her vital signs include: blood pressure 121/81 mm Hg, heart rate 77/min, and respiratory rate 15/min. Physical examination is largely negative. Given the following options, which is the most likely pathogen responsible for her presentation?

A. Campylobacter
B. Shigella (Correct Answer)
C. Salmonella
D. Clostridium difficile
E. E. coli O157:H7

Explanation: ***Shigella*** - **Shigella** infection commonly presents with sudden onset of high fever, **abdominal cramps**, and **bloody, watery diarrhea**, often with a small volume. - The patient's presentation of 4 days of abdominal pain and increasingly frequent **bloody diarrhea** with **6 episodes per day** is highly characteristic of shigellosis. *Campylobacter* - **Campylobacter** infection often manifests with **bloody diarrhea**, but it typically presents with preceding **fever** and **malaise**, followed by diarrhea that can be bloody. - While it can cause bloody diarrhea, the progression described in the patient (increasingly frequent and bloody) might be more indicative of other bacterial dysenteries. *E. coli 0157:H7* - **E. coli O157:H7** is a common cause of **hemorrhagic colitis**, characterized by **severe abdominal cramps** and **bloody diarrhea** without significant fever. - However, its association with **hemolytic uremic syndrome (HUS)**, especially in children, makes it a concern, and while it *can* cause this presentation, other options are more likely given the pure dysenteric picture. *Salmonella* - **Salmonella** gastroenteritis typically causes **non-bloody diarrhea**, fever, and abdominal cramps. - While some serotypes can cause invasive disease and bloody stool, the classic presentation is usually **watery diarrhea**. *Clostridium difficile* - **Clostridium difficile** infection is usually associated with **recent antibiotic use** or hospitalization, and commonly causes **foul-smelling, watery diarrhea**, which can occasionally be bloody in severe cases. - The patient's history does not mention recent antibiotic use, and the presentation of increasingly frequent and bloody diarrhea makes other pathogens more likely.

Question 84: An investigator studying mechanisms of acquired antibiotic resistance in bacteria conducts a study using isolated strains of Escherichia coli and Staphylococcus aureus. The E. coli strain harbors plasmid pRK212.1, which conveys resistance to kanamycin. The S. aureus strain is susceptible to kanamycin. Both bacterial strains are mixed in a liquid growth medium containing deoxyribonuclease. After incubation for 2 days and subsequent transfer to a solid medium, the S. aureus colonies show no lysis in response to the application of kanamycin. Analysis of chromosomal DNA from the kanamycin-resistant S. aureus strain does not reveal the kanamycin-resistance gene. Which of the following mechanisms is most likely responsible for this finding?

A. Transformation
B. Conjugation (Correct Answer)
C. Transduction
D. Transposition
E. Secretion

Explanation: ***Conjugation*** - The presence of **deoxyribonuclease (DNase)** in the growth medium inhibits **transformation**, ruling out the uptake of naked DNA. The transfer of the kanamycin resistance gene from a plasmid in *E. coli* to *S. aureus* in the presence of DNase strongly points to **cell-to-cell contact** via conjugation. - The resistance gene is found on a **plasmid** in *E. coli* and is transferred to *S. aureus*, resulting in kanamycin resistance without integrating into the *S. aureus* chromosome, which is characteristic of conjugative plasmid transfer. - **Key experimental clue**: DNase destroys free DNA in the medium, so the only way for genetic material to transfer is through **direct cell-to-cell contact**, which is the hallmark of conjugation. *Transformation* - This process involves the uptake of **naked DNA** from the environment by a bacterial cell, which would have been prevented by the presence of **deoxyribonuclease** in the medium. - Transformation typically results in the integration of the foreign DNA into the host cell's **chromosome** or stable maintenance as a plasmid, but DNase would degrade any free DNA before uptake could occur. *Transduction* - **Transduction** involves the transfer of genetic material via a **bacteriophage**. The scenario does not describe the presence of any phage particles, nor is there mention of viral vectors. - The resistance gene originates from a **plasmid** in *E. coli*, and transduction would require a phage capable of infecting both species, which is not mentioned in the experimental design. *Transposition* - **Transposition** is the movement of a segment of DNA from one location to another within the **same cell** (e.g., between a plasmid and chromosome). It does not explain the transfer of genetic material **between** two different bacterial cells. - While a **transposon** might carry the kanamycin resistance gene on the plasmid, transposition itself is not the mechanism for **inter-species transfer** observed in this experiment. *Secretion* - **Secretion** refers to the active release of molecules (proteins, enzymes, toxins) from a cell. It is not a mechanism for the direct transfer of **genetic material** (like a plasmid or gene) from one bacterium to another. - Genetic material is transferred through conjugation, transformation, or transduction, not by secretion pathways.

Question 85: A 55-year-old homeless man is presented to the emergency department by a group of volunteers after they found him coughing up blood during 1 of the beneficiary dinners they offer every week. His medical history is unknown as he recently immigrated from Bangladesh. He says that he has been coughing constantly for the past 3 months with occasional blood in his sputum. He also sweats a lot at nights and for the past 2 days, he has been thirsty with increased frequency of urination and feeling hungrier than usual. The respiratory rate is 30/min and the temperature is 38.6°C (101.5°F). He looks emaciated and has a fruity smell to his breath. The breath sounds are reduced over the apex of the right lung. The remainder of the physical exam is unremarkable. Biochemical tests are ordered, including a hemoglobin A1c (HbA1c) (8.5%) and chest radiography reveals cavitations in the apical region of the right lung. Which of the following immune cells is most critical in orchestrating the formation and maintenance of the granulomatous structure that led to these cavitations?

A. B lymphocytes
B. Treg lymphocytes
C. Epithelioid cells
D. Th1 lymphocytes (Correct Answer)
E. Th2 lymphocytes

Explanation: ***Th1 lymphocytes*** - The clinical picture strongly suggests **reactivation of tuberculosis** due to the cavitary lung lesions, constitutional symptoms, and likely immunocompromise from undiagnosed diabetes (HbA1c 8.5%). - **Th1 lymphocytes** are crucial for the cell-mediated immune response against **intracellular pathogens** like *Mycobacterium tuberculosis*, producing **interferon-gamma** which activates macrophages to form granulomas and contain the infection, thus preventing dissemination and contributing to cavitation. *B lymphocytes* - **B lymphocytes** primarily mediate **humoral immunity** by producing antibodies, which are less critical for controlling intracellular bacterial infections like tuberculosis. - While antibodies can play a role in modulating inflammation, they are not the primary cells involved in the **granuloma formation** and containment of *M. tuberculosis* within the lungs. *Treg lymphocytes* - **Treg lymphocytes** (regulatory T cells) primarily function to **suppress immune responses** to prevent autoimmunity and limit tissue damage. - While they can modulate the immune response in tuberculosis, their main role is not in the initial formation of **cavities** or primary defense against the pathogen, but rather in regulating the overall inflammatory process. *Epithelioid cells* - **Epithelioid cells** are **activated macrophages** that form the core of granulomas, but they are not lymphocytes; they are derived from monocytes. - They are a crucial component of the **granulomatous structure** itself, but their differentiation and activation are largely driven by cytokines produced by **Th1 lymphocytes**. *Th2 lymphocytes* - **Th2 lymphocytes** are primarily involved in immunity against **extracellular parasites** and in allergic reactions, mediating humoral responses through cytokines like **IL-4, IL-5, and IL-13**. - An effective immune response against *Mycobacterium tuberculosis* is dominated by a **Th1 cellular response**, and a prominent Th2 response is generally considered detrimental or insufficient in controlling the infection.

Question 86: A 40-year-old man presents to the office complaining of chills, fever, and productive cough for the past 24 hours. He has a history of smoking since he was 18 years old. His vitals are: heart rate of 85/min, respiratory rate of 20/min, temperature 39.0°C (102.2°F), blood pressure 110/70 mm Hg. On physical examination, there is dullness on percussion on the upper right lobe, as well as bronchial breath sounds and egophony. The plain radiograph reveals an increase in density with an alveolar pattern in the upper right lobe. Which one is the most common etiologic agent of the suspected disease?

A. Legionella pneumophila
B. Chlamydia pneumoniae
C. Mycoplasma pneumoniae
D. Streptococcus pneumoniae (Correct Answer)
E. Haemophilus influenzae

Explanation: ***Streptococcus pneumoniae*** - This patient presents with classic symptoms of **community-acquired pneumonia (CAP)**, including fever, chills, productive cough, and specific findings on physical exam (dullness, bronchial breath sounds, egophony) and chest X-ray (**lobar consolidation**). - **_Streptococcus pneumoniae_** is the most common bacterial cause of CAP worldwide, accounting for a significant percentage of cases, especially in adults. *Legionella pneumophila* - While _Legionella_ can cause severe pneumonia, it often presents with **GI symptoms** (diarrhea, nausea) and **neurological symptoms** (confusion) in addition to respiratory symptoms, which are not described here. - Risk factors typically include exposure to **contaminated water sources**, and the pneumonia can be rapidly progressive. *Chlamydia pneumoniae* - _Chlamydia pneumoniae_ typically causes a more **atypical pneumonia**, often with a more insidious onset, prolonged cough, and less severe systemic symptoms. - It usually presents as a **walking pneumonia** with milder findings on chest X-ray, unlike the clear lobar consolidation described. *Mycoplasma pneumoniae* - Like _Chlamydia pneumoniae_, _Mycoplasma pneumoniae_ is a common cause of **atypical pneumonia**, often with a gradual onset, hacking cough, and less pronounced fever. - It rarely causes the classic lobar consolidation seen in this patient and is often referred to as "walking pneumonia." *Haemophilus influenzae* - _Haemophilus influenzae_ is a significant cause of CAP, especially in patients with **underlying lung disease** (like COPD) or other comorbidities. - While certainly a possibility given the patient's smoking history, **_Streptococcus pneumoniae_** remains the overall most common cause of bacterial CAP in otherwise healthy adults.

Question 87: A 65-year-old male with a history of COPD presents to the emergency department with dyspnea, productive cough, and a fever of 40.0°C (104.0°F) for the past 2 days. His respiratory rate is 20/min, blood pressure is 125/85 mm Hg, and heart rate is 95/min. A chest X-ray is obtained and shows a right lower lobe infiltrate. Sputum cultures are pending and he is started on antibiotics. The patient has not received any vaccinations in the last 20 years. The physician discusses with him the importance of getting a vaccine that can produce immunity via which of the following mechanisms?

A. T cell-dependent B cell response (Correct Answer)
B. Natural killer cell response
C. Mast cell degranulation response
D. Complement activation response
E. T cell-independent B cell response

Explanation: ***T cell-dependent B cell response*** - The patient, suffering from **community-acquired pneumonia** frequently caused by *Streptococcus pneumoniae*, would benefit from the **pneumococcal polysaccharide conjugate vaccine (PCV13)**. - PCV13 contains **polysaccharide antigens conjugated to a protein carrier**, which allows for a **T cell-dependent B cell response**, leading to better immunogenicity, **memory cell formation**, and broader protection, especially in adults. *Natural killer cell response* - **Natural killer (NK) cells** are part of the **innate immune system** and are primarily involved in surveillance against infected or cancerous cells, not primary antibody production for bacterial infections. - While NK cells play a role in overall immunity, they are not the primary mechanism by which **vaccines induce long-term protective immunity** against encapsulated bacteria like *S. pneumoniae*. *Mast cell degranulation response* - **Mast cell degranulation** is primarily associated with **allergic reactions** and defense against parasites, releasing mediators like histamine. - This mechanism is not involved in generating protective immunity against bacterial infections through vaccination; rather, it can be a harmful immune response in certain contexts. *Complement activation response* - **Complement activation** is a crucial part of both innate and adaptive immunity, aiding in pathogen clearance through opsonization, lysis, and inflammation. - While antibodies produced by vaccination can activate complement, the vaccine itself doesn't directly induce immunity via complement; rather, complement acts downstream of the **antibody response**. *T cell-independent B cell response* - The **pneumococcal polysaccharide vaccine (PPSV23)**, made from **unconjugated polysaccharides**, induces a T cell-independent B cell response, which is less robust in adults and does not produce memory cells. - This response is characterized by poorer immunogenicity in young children and elderly individuals, and a lack of **affinity maturation** or **immunological memory** compared to T cell-dependent responses.

Question 88: One week after starting amoxicillin for sinusitis, a 4-year-old girl is brought to the emergency department with fever, rash, and myalgia. She has been hospitalized multiple times for recurrent streptococcal pneumonia and meningitis. She appears tired. Examination shows a diffuse urticarial rash. Her antibiotic is discontinued. Which of the following is the most likely underlying mechanism for her recurrent infections?

A. Impaired opsonization (Correct Answer)
B. Absence of IgA antibodies
C. Defective superoxide production
D. Impaired leukocyte adhesion
E. Accumulation of bradykinin

Explanation: ***Impaired opsonization*** - Recurrent severe bacterial infections, particularly with **encapsulated organisms** like *Streptococcus pneumoniae*, suggest a defect in the **complement system** or **antibody production**, which are crucial for opsonization. - The pattern of recurrent **pneumonia and meningitis** with *Streptococcus pneumoniae* is highly suggestive of **complement deficiency** (particularly C3 or alternative pathway defects), which impairs opsonization and clearance of encapsulated bacteria. *Absence of IgA antibodies* - Selective IgA deficiency typically presents with recurrent **mucosal infections** (e.g., upper respiratory, gastrointestinal), but usually not severe, disseminated bacterial infections like meningitis. - While IgA is important for mucosal immunity, its absence alone does not explain generalized recurrent **streptococcal pneumonia** and **meningitis** as the primary underlying cause. *Defective superoxide production* - Defective superoxide production, as seen in **chronic granulomatous disease (CGD)**, leads to recurrent infections with **catalase-positive organisms** (e.g., *Staphylococcus aureus*, *Aspergillus*). - The patient's history of recurrent **streptococcal infections** (which are catalase-negative) and lack of **granuloma formation** makes CGD less likely. *Impaired leukocyte adhesion* - Impaired leukocyte adhesion, characteristic of **leukocyte adhesion deficiency (LAD)**, leads to recurrent bacterial infections, **impaired wound healing**, and **delayed umbilical cord separation**. - While recurrent infections are present, the clinical picture does not specifically mention impaired wound healing or delayed cord separation, and the specific organisms involved (streptococcal) are not pathognomonic for LAD. *Accumulation of bradykinin* - Accumulation of bradykinin leads to **hereditary angioedema**, characterized by recurrent episodes of localized **swelling** without urticaria. - This condition does not primarily manifest with recurrent bacterial infections or a diffuse urticarial rash in response to medication, making it an unlikely underlying mechanism for the patient's symptoms.

Question 89: A 32-year-old man is brought to the physician by his wife for a 3-day history of fever, headaches, and myalgias. He returned from a camping trip in Oklahoma 10 days ago. He works as a computer salesman. His temperature is 38.1°C (100.6°F). Neurologic examination shows a sustained clonus of the right ankle following sudden passive dorsiflexion. He is disoriented to place and time but recognizes his wife. Laboratory studies show a leukocyte count of 1,700/mm3 and a platelet count of 46,000/mm3. A peripheral blood smear shows monocytes with intracytoplasmic morulae. Which of the following is the most likely causal organism?

A. Coxiella burnetii
B. Rickettsia rickettsii
C. Anaplasma phagocytophilum
D. Borrelia burgdorferi
E. Ehrlichia chaffeensis (Correct Answer)

Explanation: ***Correct: Ehrlichia chaffeensis*** - The presence of **intracytoplasmic morulae** in **monocytes** is a pathognomonic sign for *Ehrlichia chaffeensis* infection, which causes **human monocytic ehrlichiosis**. - The patient's symptoms of **fever, headache, myalgias, thrombocytopenia, leukopenia**, and the history of a **camping trip** in an endemic area (Oklahoma) are highly consistent with ehrlichiosis. *Incorrect: Coxiella burnetii* - This bacterium causes **Q fever**, characterized by fever, headache, and atypical pneumonia, but it does **not cause intracytoplasmic morulae** in monocytes or frequently lead to the degree of leukopenia and thrombocytopenia seen here. - While it can be acquired from environments, the **microscopic findings** rule it out in this specific case. *Incorrect: Rickettsia rickettsii* - This organism causes **Rocky Mountain spotted fever**, which presents with fever, headache, myalgias, and a characteristic **rash** that is usually present on the palms and soles, none of which are mentioned here. - It does not form **intracytoplasmic morulae** in monocytes. *Incorrect: Anaplasma phagocytophilum* - This bacterium causes **human granulocytic anaplasmosis**, which is clinically similar to ehrlichiosis but forms **morulae in granulocytes** (neutrophils), not monocytes. - The peripheral blood smear specifically identifies morulae in **monocytes**, directing towards *Ehrlichia*. *Incorrect: Borrelia burgdorferi* - This spirochete causes **Lyme disease**, characterized by an **expanding erythematous rash (erythema migrans)**, fever, and musculoskeletal symptoms, but it does not cause leukopenia or thrombocytopenia. - It does not produce **morulae** in any blood cells.

Question 90: A 7-month-old Caucasian male presents with recurrent sinusitis and pharyngitis. The parents say that the child has had these symptoms multiple times in the past couple of months and a throat swab sample reveals the presence of Streptococcus pneumoniae. Upon workup for immunodeficiency it is noted that serum levels of immunoglobulins are extremely low but T-cell levels are normal. Which of the following molecules is present on the cells that this patient lacks?

A. CD8
B. CD3
C. CD4
D. NKG2D
E. CD19 (Correct Answer)

Explanation: ***CD19*** - The patient's symptoms of **recurrent bacterial infections** (sinusitis, pharyngitis with *Streptococcus pneumoniae*) and **extremely low serum immunoglobulin levels** despite normal T-cell levels are highly indicative of **X-linked agammaglobulinemia (XLA)**. - XLA is characterized by the absence of **mature B cells**, which are responsible for antibody production. **CD19** is a pan B-cell marker, universally expressed on B cells from their earliest stages in the bone marrow (pro-B cells) through to mature circulating B cells, identifying the cell population that is lacking in this patient. *CD8* - **CD8** is a co-receptor expressed on **cytotoxic T cells**, involved in recognizing antigens presented by MHC class I molecules. - The patient's T-cell levels are reported as **normal**, indicating that both CD4+ and CD8+ T-cell populations are likely present. *CD3* - **CD3** is a complex of proteins expressed on the surface of all **T lymphocytes** (both helper and cytotoxic T cells) as part of the T-cell receptor (TCR) complex. - Since the patient has **normal T-cell levels**, cells expressing CD3 are present and functional. *CD4* - **CD4** is a co-receptor primarily expressed on **helper T cells**, which recognize antigens presented by MHC class II molecules. - Similar to CD8 and CD3, **normal T-cell levels** suggest that CD4+ T cells are present in this patient. *NKG2D* - **NKG2D** is an activating receptor expressed primarily on **NK cells** (natural killer cells), as well as on subsets of CD8+ T cells and gamma-delta T cells. - While NK cells play a role in innate immunity, their absence or dysfunction would not directly explain the specific deficiency of **immunoglobulins** and recurrent bacterial infections seen here, which points to a B-cell defect.

Question 91: A 2-year-old boy has a history of recurrent bacterial infections, especially of his skin. When he has an infection, pus does not form. His mother reports that, when he was born, his umbilical cord took 5 weeks to detach. He is ultimately diagnosed with a defect in a molecule in the pathway that results in neutrophil extravasation. Which of the following correctly pairs the defective molecule with the step of extravasation that molecule affects?

A. E-selectin; transmigration
B. LFA-1 (integrin); tight adhesion (Correct Answer)
C. ICAM-1; margination
D. E-selectin; tight adhesion
E. PECAM-1; transmigration

Explanation: ***LFA-1 (integrin); tight adhesion*** - This patient's symptoms (recurrent bacterial infections, lack of pus formation, and delayed umbilical cord separation) are classic for **Leukocyte Adhesion Deficiency type 1 (LAD-1)**. - **LAD-1** is caused by a defect in the **CD18 subunit** of **β2 integrins**, including **LFA-1** and **Mac-1**, which are crucial for the **tight adhesion** of neutrophils to endothelial cells. *E-selectin; transmigration* - **E-selectin** mediates the initial **rolling** of leukocytes along the endothelial surface, not transmigration. - A defect in E-selectin would impair rolling, but the primary defect in LAD-1 is in tight adhesion. *ICAM-1; margination* - **ICAM-1** (Intercellular Adhesion Molecule-1) is an endothelial ligand that binds to integrins on leukocytes, facilitating **tight adhesion** and transmigration, not margination. - **Margination** refers to the movement of leukocytes to the periphery of the blood vessel lumen. *E-selectin; tight adhesion* - **E-selectin** is involved in the initial **rolling** phase of extravasation by binding to sialyl Lewis X on leukocytes. - It does not primarily mediate **tight adhesion**, which is facilitated by integrins binding to ICAM-1. *PECAM-1; transmigration* - **PECAM-1** (Platelet Endothelial Cell Adhesion Molecule-1) is primarily involved in **transmigration** (diapedesis), where leukocytes pass between endothelial cells. - While important for extravasation, the characteristic findings of LAD-1 point to a defect earlier in the pathway, specifically tight adhesion.

Question 92: A 35-year-old man from Thailand presents with low-grade fever, chronic cough, and night sweats for 3 months. He describes the cough as productive and producing white sputum that is sometimes streaked with blood. He also says he has lost 10 lb in the last 3 months. Past medical history is unremarkable. The patient denies any smoking history, alcohol, or recreational drug use. The vital signs include blood pressure 115/75 mm Hg, heart rate 120/min, respiratory rate 20/min, and temperature 36.6℃ (97.8℉). On physical examination, the patient is ill-looking and thin with no pallor or jaundice. Cardiopulmonary auscultation reveals some fine crackles in the right upper lobe. A chest radiograph reveals a right upper lobe homogeneous density. Which of the following tests would be most helpful in making a definitive diagnosis of active infection in this patient?

A. PPD test
B. Silver stain
C. Ziehl-Neelsen stain (Correct Answer)
D. Gram stain
E. Interferon-gamma assay

Explanation: ***Ziehl-Neelsen stain*** - The patient's symptoms (low-grade fever, chronic cough with white/bloody sputum, night sweats, weight loss) and chest X-ray findings (right upper lobe homogeneous density) are highly suggestive of **active tuberculosis (TB)**, especially given his origin from Thailand (a country with a high TB burden). - The **Ziehl-Neelsen stain** (acid-fast stain) directly visualizes **acid-fast bacilli** (AFB) like *Mycobacterium tuberculosis* in sputum, providing a rapid and definitive diagnosis of active infection. *PPD test* - A **PPD test** (tuberculin skin test) indicates exposure to *Mycobacterium tuberculosis* but cannot differentiate between **latent TB infection** and **active disease**. - A positive PPD can occur in individuals previously exposed or vaccinated with BCG, offering no direct evidence of current active infection. *Silver stain* - **Silver stain** (e.g., Gomori methenamine silver) is used to identify fungal organisms like *Pneumocystis jirovecii* or certain bacteria, such as *Legionella pneumophila*. - It is not the primary stain for diagnosing tuberculosis, which requires detection of acid-fast bacilli. *Gram stain* - **Gram stain** is used to classify bacteria based on their cell wall properties (Gram-positive or Gram-negative). - *Mycobacterium tuberculosis* has a unique **mycolic acid-rich cell wall** that makes it resistant to Gram staining and requires acid-fast staining for visualization. *Interferon-gamma assay* - An **interferon-gamma release assay (IGRA)**, like the Quantiferon-TB Gold test, detects exposure to *Mycobacterium tuberculosis* and is used to diagnose **latent TB infection**. - Similar to the PPD test, it cannot distinguish between latent infection and **active disease**, and a positive result requires further investigation for active TB.

Question 93: A 46-year-old woman presents to her primary care physician with complaints of increasing left upper quadrant discomfort. She has a known history of type 1 Gaucher disease. On physical examination, her spleen is palpable 8 cm below the costal margin. Routine laboratory work reveals severe pancytopenia. After consultation with the patient on the risks of her condition, the patient decides to undergo a splenectomy. Which of the following is more likely to occur as a consequence of splenectomy in this patient?

A. Thrombocytopenia
B. Leukopenia
C. Pneumococcal septicemia (Correct Answer)
D. Staphylococcal septicemia
E. Anemia

Explanation: ***Pneumococcal septicemia*** - Patients who undergo splenectomy are at significantly increased risk of **overwhelming post-splenectomy infection (OPSI)**, particularly from **encapsulated bacteria** like *Streptococcus pneumoniae*. - The spleen plays a crucial role in filtering encapsulated bacteria and producing opsonizing antibodies, and its removal compromises this immune function. *Thrombocytopenia* - **Thrombocytopenia** is typically a symptom *before* splenectomy in Gaucher disease due to hypersplenism. - After splenectomy, the platelet count often **increases**, not decreases, due to the removal of the organ that sequesters platelets and destroys them. *Leukopenia* - **Leukopenia** (specifically neutropenia) is a pre-existing condition in severe Gaucher disease due to hypersplenism and bone marrow involvement. - Post-splenectomy, the white blood cell count, particularly neutrophils, generally **increases** as the sequestration and destruction in the spleen are eliminated. *Staphylococcal septicemia* - While *Staphylococcus* can cause serious infections, it is **not the primary pathogen** associated with OPSI in asplenic patients. - Encapsulated bacteria like *Streptococcus pneumoniae* are the most common and dangerous cause of post-splenectomy sepsis. *Anemia* - **Anemia** is a common finding in Gaucher disease due to hypersplenism and bone marrow infiltration. - Splenectomy typically **improves** anemia by removing the site of red blood cell destruction and reducing abnormal cytokine production that inhibits erythropoiesis.

Question 94: A 45-year-old man visits the office with complaints of severe pain with urination for 5 days. In addition, he reports having burning discomfort and itchiness at the tip of his penis. He is also concerned regarding a yellow-colored urethral discharge that started a week ago. Before his symptoms began, he states that he had sexual intercourse with multiple partners at different parties organized by the hotel he was staying at. Physical examination shows edema and erythema concentrated around the urethral meatus accompanied by a mucopurulent discharge. Which of the following diagnostic tools will best aid in the identification of the causative agent for his symptoms?

A. Nucleic acid amplification tests (NAATs) (Correct Answer)
B. Urethral biopsy
C. Tzanck smear
D. Leukocyte esterase dipstick test
E. Gram stain

Explanation: ***Nucleic acid amplification tests (NAATs)*** - NAATs are the **most sensitive and specific diagnostic tools** for detecting common sexually transmitted infections (STIs) like **gonorrhea** and **chlamydia**, which present with urethral discharge, dysuria, and itching. - They can identify the **genetic material** of the causative organisms directly from urine samples or urethral swabs, making them highly effective even with low bacterial loads. *Urethral biopsy* - A urethral biopsy is an **invasive procedure** generally reserved for investigating conditions like **strictures, tumors, or chronic inflammatory diseases** when other diagnostic methods are inconclusive. - It is not a primary diagnostic tool for acute urethritis suspected to be an STI, as it carries risks and is unnecessary given the availability of less invasive options. *Tzanck smear* - The Tzanck smear is primarily used for diagnosing **herpes simplex virus (HSV) infections** by looking for multinucleated giant cells and intranuclear inclusions. - While HSV can cause genital lesions, it typically does not present as a primary symptom of mucopurulent urethral discharge and dysuria without visible vesicles or ulcers, making it less likely in this scenario. *Leukocyte esterase dipstick test* - A leukocyte esterase dipstick test detects the presence of **white blood cells** in urine, indicating inflammation or infection in the urinary tract. - While it can suggest urethritis, it is **not specific for the causative agent** and merely indicates inflammation, requiring further specific testing to identify the pathogen. *Gram stain* - A Gram stain of urethral discharge can rapidly identify Gram-negative intracellular diplococci suggestive of **gonorrhea** (Neisseria gonorrhoeae). - However, its sensitivity for gonorrhea is lower than NAATs, especially in asymptomatic cases or for detecting other common causes of urethritis like **Chlamydia trachomatis**, which are not visible on Gram stain.

Question 95: A 7-month-old girl is brought to the hospital by her mother, who complains of a lesion on the infant’s labia for the past 5 days. The lesion is 2 x 2 cm in size and red in color with serosanguinous fluid oozing out of the right labia. The parents note that the girl has had a history of recurrent bacterial skin infections with no pus but delayed healing since birth. She also had delayed sloughing of the umbilical cord at birth. Complete blood count results are as follows: Neutrophils on admission Leukocytes 19,000/mm3 Neutrophils 83% Lymphocytes 10% Eosinophils 1% Basophils 1% Monocytes 5% Hemoglobin 14 g/dL Which of the following compounds is most likely to be deficient in this patient?

A. Selectin
B. TNF-alpha
C. Cellular adhesion molecule
D. Integrin subunit (Correct Answer)
E. vWF

Explanation: **Integrin subunit** ✓ - The patient exhibits features consistent with **Leukocyte Adhesion Deficiency Type 1 (LAD-1)**, characterized by recurrent bacterial infections without pus formation, delayed umbilical cord separation, and **leukocytosis with neutrophil predominance** (due to inability of neutrophils to exit circulation and migrate to tissues). - LAD-1 is caused by a defect in the **CD18 integrin β2 subunit**, which is essential for the function of β2 integrins (e.g., LFA-1, MAC-1, CR3) involved in firm adhesion and transmigration of leukocytes across the endothelium. - The absence of pus despite bacterial infections is explained by the inability of neutrophils to migrate to infection sites. *Selectin* - **Selectins** (E-selectin, P-selectin, L-selectin) mediate the initial **rolling** phase of leukocyte adhesion to the endothelium. - Defects in selectins (specifically fucose metabolism) are associated with **LAD-2 (Leukocyte Adhesion Deficiency Type 2)**, which presents with similar infection susceptibility but typically includes additional features like growth retardation, intellectual disability, and the Bombay blood phenotype. *TNF-alpha* - **TNF-alpha** is a pro-inflammatory cytokine that upregulates adhesion molecules on endothelial cells and activates immune responses. - While TNF-alpha deficiency or blockade can increase infection susceptibility, it does not explain the specific constellation of delayed umbilical cord separation, persistent neutrophilic leukocytosis, and infections without pus formation seen in this patient. *Cellular adhesion molecule* - This is a broad, non-specific term encompassing multiple adhesion molecule families (integrins, selectins, immunoglobulin superfamily members, cadherins). - While integrins are indeed cellular adhesion molecules, **"integrin subunit"** is the specific and precise answer for LAD-1, as the defect involves the β2 integrin subunit (CD18). *vWF* - **Von Willebrand Factor (vWF)** is crucial for platelet adhesion to damaged endothelium and serves as a carrier protein for factor VIII. - Deficiency causes **Von Willebrand disease**, presenting with mucocutaneous bleeding (epistaxis, easy bruising, menorrhagia), not recurrent bacterial infections, delayed wound healing, or the immunologic abnormalities observed in this patient.

Question 96: A 32-year-old woman comes to the emergency department with a 2-day history of abdominal pain and diarrhea. She has had about 8 voluminous stools per day, some of which were bloody. She visited an international food festival three days ago. She takes no medications. Her temperature is 39.5°C (103.1°F), pulse is 90/min, and blood pressure is 110/65 mm Hg. Examination shows a tender abdomen, increased bowel sounds, and dry mucous membranes. Microscopic examination of the stool shows polymorphonuclear leukocytes. Stool culture results are pending. Which of the following most likely caused the patient's symptoms?

A. Home-canned vegetables
B. Yogurt dip
C. Reheated rice
D. Toxic mushrooms
E. Omelette (Correct Answer)

Explanation: **Omelette** - The symptoms, including **bloody diarrhea**, fever, and exposure to an international food festival suggest a **bacterial infection**, likely from contaminated eggs (e.g., **Salmonella**). - The presence of **polymorphonuclear leukocytes** in the stool indicates an **invasive bacterial infection**, consistent with salmonellosis. *Home-canned vegetables* - Poorly preserved home-canned vegetables are a classic cause of **botulism**, which presents with **neurological symptoms** (e.g., flaccid paralysis) and is not characterized by bloody diarrhea or fever. - While it can cause gastrointestinal upset, bloody stools are not typical, and the primary concern is neurotoxicity due to **Clostridium botulinum toxin**. *Yogurt dip* - Yogurt is a dairy product, and contamination typically leads to **non-bloody diarrhea** and vomiting, often caused by bacteria like *Staphylococcus aureus* or *Bacillus cereus* producing enterotoxins. - The symptoms would likely be less severe and lack the invasive features (bloody stools, fever, PMNs) seen in this patient. *Reheated rice* - Reheated rice is commonly associated with **Bacillus cereus** food poisoning, which typically causes either an emetic (vomiting) or diarrheal syndrome. - The diarrhea caused by *Bacillus cereus* is usually **watery and non-bloody**, and it rarely presents with significant fever or invasive features like polymorphonuclear leukocytes in stool. *Toxic mushrooms* - Mushroom poisoning can present with a wide range of symptoms, including gastrointestinal distress (vomiting, diarrhea), but the presentation varies greatly depending on the mushroom species. - **Bloody diarrhea** with fever and polymorphonuclear leukocytes in stool is not a typical hallmark of common toxic mushroom ingestions, which often involve hepatotoxicity or neurotoxicity.

Question 97: A 24-year-old female comes to the physician because of flu-like symptoms and a new rash for 2 days. She denies contacts with sick individuals or recent travel abroad, but recently went camping in Vermont. Vital signs are within normal limits. Examination of the lateral right thigh shows a circular red ring with central clearing. Which of the following is the natural reservoir of the pathogen responsible for this patient's symptoms?

A. Rat
B. Rabbit
C. Tick
D. Mouse (Correct Answer)
E. Flea

Explanation: ***Mouse*** - The patient's symptoms, including **flu-like illness** and a **circular red rash with central clearing** (erythema migrans) after camping in Vermont, are classic for **Lyme disease**. - The causative agent, *Borrelia burgdorferi*, is primarily maintained in **white-footed mice** (genus *Peromyscus*) in its natural reservoir during its larval and nymphal stages. *Rat* - While **rats** can carry and transmit various diseases, they are not the primary natural reservoir for *Borrelia burgdorferi*, the pathogen responsible for Lyme disease. - Diseases associated with rats often include **leptospirosis** and **plague**, which present with different clinical pictures. *Rabbit* - **Rabbits** are known reservoirs for diseases like **tularemia** (*Francisella tularensis*), which can cause fever, skin lesions, and lymphadenopathy, but typically not the characteristic **erythema migrans** rash. - They are not a significant natural reservoir for *Borrelia burgdorferi*. *Tick* - The **tick** (specifically *Ixodes scapularis* or **deer tick**) is the **vector** that transmits *Borrelia burgdorferi* to humans, not the natural reservoir. - The tick acquires the bacteria from infected animal hosts such as mice and deer. *Flea* - **Fleas** are vectors for diseases such as **bubonic plague** (*Yersinia pestis*) and **endemic typhus** (*Rickettsia typhi*), which do not manifest with erythema migrans. - They are not involved in the transmission or natural history of **Lyme disease**.

Question 98: A 13-month-old boy is referred to an immunologist with recurrent otitis media, bacterial sinus infections, and pneumonia, which began several months earlier. He is healthy now, but the recurrent nature of these infections are troubling to his parents and they are hoping to find a definitive cause. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. The patient has five older siblings, but none of them had similar recurrent illnesses. Clinical pathology results suggest very low levels of serum immunoglobulin. As you discuss options for diagnosis with the patient’s family, which of the following tests should be performed next?

A. CSF gram staining
B. Urine protein screening
C. Stool cultures
D. Flow cytometry (Correct Answer)
E. Genetic analysis

Explanation: ***Flow cytometry*** - Flow cytometry is essential for evaluating **lymphocyte subsets** (B cells, T cells, NK cells) and their maturation, which is crucial for diagnosing **primary immunodeficiencies** like X-linked agammaglobulinemia (XLA). - Given the history of recurrent bacterial infections and **very low serum immunoglobulin levels**, assessing B cell numbers and T cell populations would directly help identify defects in humoral immunity. *CSF gram staining* - **CSF gram staining** is used to diagnose **bacterial meningitis** at the time of an active infection. - The patient is currently healthy, and the test would not identify the underlying cause of recurrent infections or low immunoglobulin levels. *Urine protein screening* - **Urine protein screening** is used to detect **kidney disease** or other conditions causing proteinuria. - It is not relevant to investigating recurrent bacterial infections or low serum immunoglobulin levels, which point towards an immune system defect. *Stool cultures* - **Stool cultures** are performed to identify **gastrointestinal infections** (e.g., bacterial, parasitic). - While infections can occur in immunodeficient patients, this test is not a primary diagnostic tool for the underlying **immunodeficiency** causing recurrent otitis media, sinus infections, and pneumonia. *Genetic analysis* - **Genetic analysis** can confirm certain **primary immunodeficiency diagnoses** once specific defects are suspected (e.g., mutations in *BTK* for XLA). - However, flow cytometry is typically the next step to broadly characterize the immune cell populations and narrowed down differential diagnoses before proceeding with targeted genetic testing.

Question 99: A 55-year-old man who recently immigrated to the United States from Azerbaijan comes to the physician because of a 6-week history of recurrent fever, progressive cough with bloody streaks, fatigue, and a 3.6-kg (8-lb) weight loss. He has poorly-controlled type 2 diabetes mellitus treated with insulin. An x-ray of the chest shows a cavitary lesion of the posterior apical segment of the right upper lobe with consolidation of the surrounding parenchyma. He is started on a treatment regimen with a combination of drugs. A culture of the sputum identifies a causal pathogen that is resistant to a drug that alters the metabolism of pyridoxine. Which of the following is the most likely mechanism of resistance to this drug?

A. Increased production of arabinosyl transferase
B. Mutation in genes encoding RNA polymerase
C. Changed amino acid composition of DNA gyrase
D. Impaired conversion to pyrazinoic acid
E. Decreased production of catalase-peroxidase (Correct Answer)

Explanation: ***Decreased production of catalase-peroxidase*** - The drug described as altering the metabolism of pyridoxine is **isoniazid**, which requires activation by the catalase-peroxidase enzyme **KatG** in *Mycobacterium tuberculosis*. - Resistance to isoniazid often develops through **mutations or deletions in the *katG* gene**, leading to decreased or absent catalase-peroxidase activity and thus impaired activation of the drug. *Increased production of arabinosyl transferase* - This mechanism is associated with resistance to **ethambutol**, which inhibits **arabinosyl transferase**, an enzyme crucial for cell wall synthesis. - Increased production of this enzyme would counteract the drug's effect, but it is not relevant to the drug that alters pyridoxine metabolism (isoniazid). *Mutation in genes encoding RNA polymerase* - This mechanism is responsible for resistance to **rifampin**, which targets the **bacterial DNA-dependent RNA polymerase**. - Mutations in the *rpoB* gene prevent rifampin from binding, but this drug does not alter pyridoxine metabolism. *Changed amino acid composition of DNA gyrase* - This mechanism is characteristic of resistance to **fluoroquinolones** (e.g., ciprofloxacin, levofloxacin), which inhibit **DNA gyrase** and topoisomerase IV. - Alterations in the drug-binding sites of these enzymes reduce the efficacy of fluoroquinolones, a different class of antibiotics from isoniazid. *Impaired conversion to pyrazinoic acid* - This is the primary mechanism of resistance to **pyrazinamide**, which is a prodrug that needs to be converted to its active form, **pyrazinoic acid**, by the bacterial enzyme **pyrazinamidase (PncA)**. - Resistance typically involves mutations in the *pncA* gene, but pyrazinamide is not the drug that alters pyridoxine metabolism.

Question 100: A 31-year-old woman presents to the emergency department with a history of fever and vomiting for 2 days and severe headache for a day. Past medical history is significant for migraine diagnosed 10 years ago, but she reports that her current headache is different. She describes the pain as generalized, dull, continuous, severe in intensity, and exacerbated by head movements. Physical examination reveals a blood pressure of 110/76 mm Hg and a temperature of 39.1°C (102.4°F). The patient is awake but in great distress due to pain. A pink-purple petechial rash covers her chest and legs. Extraocular movements are normal. She complains of neck pain and asks you to turn off the lights. Muscle strength is normal in all 4 limbs. Fundoscopic examination is normal. Baseline laboratory investigations are shown: Laboratory test Sodium 145 mEq/L Potassium 3.2 mEq/L Glucose 87 mg/dL Creatinine 1.0 mg/dL White blood cell count 18,900/mm3 Hemoglobin 13.4 g/dL Platelets 165,000/mm3 INR 1.1 Aerobic and anaerobic blood cultures are taken and empiric antibiotics are started. A lumbar puncture is performed. Which of the following cerebrospinal fluid (CSF) findings are expected in this patient?

A. CSF: WBC 640 cells/mm3, lymphocytic predominant, protein 180 mg/dL, glucose 24 mg/dL
B. CSF: WBC 8,500 cells/mm3, neutrophil predominant, protein 112 mg/dL, glucose 15 mg/dL (Correct Answer)
C. CSF: WBC 145 cells/mm3, lymphocytic predominant, protein 42 mg/dL, glucose 60 mg/dL
D. CSF: WBC 4 cells/mm3, protein 35 mg/dL, glucose 66 mg/dL
E. CSF: WBC 2 cells/mm3, protein 142 mg/dL, glucose 70 mg/dL

Explanation: ***CSF: WBC 8,500 cells/mm3, neutrophil predominant, protein 112 mg/dL, glucose 15 mg/dL*** - The clinical presentation with **fever, severe headache, neck pain (photophobia indicated by asking to turn off lights), and petechial rash** is highly suggestive of **bacterial meningitis**, specifically meningococcal meningitis given the rash. - **Bacterial meningitis** characteristically shows a **markedly elevated CSF white blood cell count (WBC) with neutrophil predominance**, **elevated protein**, and **very low glucose** due to bacterial consumption. *CSF: WBC 640 cells/mm3, lymphocytic predominant, protein 180 mg/dL, glucose 24 mg/dL* - While this CSF profile shows elevated protein and low glucose, the **lymphocytic predominance** typically points towards **viral meningitis** or other chronic inflammatory conditions, not acute bacterial meningitis. - The high WBC count is more typical of bacterial meningitis, but the cell type is inconsistent with the typical presentation of acute bacterial infection. *CSF: WBC 145 cells/mm3, lymphocytic predominant, protein 42 mg/dL, glucose 60 mg/dL* - This CSF profile suggests **viral meningitis**, characterized by a **moderately elevated WBC count with lymphocytic predominance**, relatively normal or mildly elevated protein, and normal glucose. - The patient's severe presentation with high fever, rash, and marked leukocytosis is more consistent with a bacterial process. *CSF: WBC 4 cells/mm3, protein 35 mg/dL, glucose 66 mg/dL* - This CSF profile represents **normal findings**, with a very low WBC count, normal protein, and normal glucose. - These findings would not explain the patient's severe symptoms, high fever, and suspected meningitis. *CSF: WBC 2 cells/mm3, protein 142 mg/dL, glucose 70 mg/dL* - This profile shows normal WBCs and glucose but **elevated protein**. This combination can be seen in conditions like **Guillain-Barré syndrome** (cytoalbuminologic dissociation) or certain tumors. - It does not fit the acute infectious picture of meningitis with fever and systemic inflammation.

Question 101: A 4-year-old boy is brought to the clinic by his mother with a history of fever for the past 3 days, yellow nasal discharge, and a severe earache in the right ear. He has no prior history of ear infections and is otherwise healthy. The physician suspects that the infectious agent is Streptococcus pneumoniae and prescribes the appropriate treatment. Which of the following is true about the mechanism of antigen processing in this example?

A. The pathway involved allows for recognition of extracellular antigens. (Correct Answer)
B. The target cell involved is a CD8+ T cell.
C. The pathway involved allows for recognition of intracellular antigens.
D. The antigen degradation occurs via the proteasome.
E. The antigen is directly bound to the MHC I.

Explanation: ***The pathway involved allows for recognition of extracellular antigens.*** - *Streptococcus pneumoniae* is an **extracellular bacterium**, meaning it replicates outside of host cells. - Antigens from extracellular pathogens are typically processed via the **MHC class II pathway**, which presents peptides to CD4+ helper T cells. *The target cell involved is a CD8+ T cell.* - **CD8+ T cells** (cytotoxic T lymphocytes) primarily recognize antigens presented on **MHC class I molecules**, which are generally associated with intracellular pathogens or cancerous cells. - In this case, the immune response is directed against an extracellular bacterium, primarily calling for **CD4+ helper T cells**. *The pathway involved allows for recognition of intracellular antigens.* - The processing of **intracellular antigens** involves the **MHC class I pathway**, leading to presentation to CD8+ T cells. - *Streptococcus pneumoniae* is an **extracellular pathogen**, and its antigens are primarily recognized through the MHC class II pathway. *The antigen degradation occurs via the proteasome.* - **Proteasomal degradation** is a key step in the **MHC class I pathway**, where intracellular proteins are broken down into peptides. - For **extracellular antigens** like those from *S. pneumoniae*, degradation typically occurs in **acidified endosomes or lysosomes**. *The antigen is directly bound to the MHC I.* - Antigens are **never directly bound to MHC molecules**; they first undergo processing (degradation into peptides) before binding to a specific MHC molecule. - For extracellular pathogens, processed peptides bind to **MHC class II molecules**, not MHC class I.

Question 102: A 22-year-old woman is brought to the emergency department because of diplopia, slurred speech, progressive upper extremity weakness, and difficulty swallowing for the past several hours. She had mild abdominal pain that resolved spontaneously after returning from her father's farm yesterday. Her temperature is 37°C (98.6°F), respirations are 11/min and labored, and blood pressure is 110/70 mm Hg. Examination shows bilateral nystagmus and ptosis. The pupils are dilated and not reactive to light or accommodation. Muscle strength of the facial muscles and bilateral upper extremities is decreased. Which of the following is the strongest risk factor for this patient's condition?

A. Skin bite by Ixodes tick
B. Lack of immunization with polysaccharide fragments
C. Exposure to bacterial spores
D. Oral ingestion of preformed toxin (Correct Answer)
E. Gastroenteritis caused by comma-shaped rod

Explanation: ***Oral ingestion of preformed toxin*** - This patient presents with symptoms highly suggestive of **botulism**, including **sudden onset of diplopia**, **slurred speech**, **descending paralysis (upper extremity weakness, difficulty swallowing)**, **labored breathing**, **bilateral nystagmus**, **ptosis**, and **dilated, unreactive pupils**. These symptoms are characteristic of a **neurotoxin** that blocks acetylcholine release at the neuromuscular junction. - The history of mild abdominal pain after returning from a farm and the rapid progression of symptoms point to **foodborne botulism**, which is caused by the **ingestion of preformed botulinum toxin**, often from improperly canned or preserved foods. *Skin bite by Ixodes tick* - A tick bite by *Ixodes* species is associated with **Lyme disease** and **tick-borne encephalitis**, neither of which typically presents with the rapid, descending, symmetrical paralysis and prominent cranial nerve involvement seen here. - While **tick paralysis** exists, it is a rare peripheral neuropathy and usually results in ascending paralysis, beginning in the lower extremities, which is distinct from this patient's descending pattern. *Lack of immunization with polysaccharide fragments* - Immunization with polysaccharide fragments is characteristic of vaccines against encapsulated bacteria like *Streptococcus pneumoniae* or *Haemophilus influenzae type b*. - A lack of such immunization would predispose to localized or systemic bacterial infections but not to the acute neurological syndrome described. *Exposure to bacterial spores* - **Exposure to bacterial spores** is relevant for infant botulism (ingestion of *Clostridium botulinum* spores which then germinate and produce toxin in the gut) or wound botulism. - The patient's age (22) and the rapid onset of symptoms within hours after potential exposure to contaminated food make **foodborne botulism** (preformed toxin) more likely than spore germination. *Gastroenteritis caused by comma-shaped rod* - A **comma-shaped rod** typically refers to *Vibrio cholerae* or *Campylobacter jejuni*, which cause acute gastroenteritis. - While gastroenteritis can sometimes precede neurological manifestations in rare cases (e.g., Guillain-Barré syndrome after *Campylobacter* infection), the described neurological syndrome (acute, descending paralysis with cranial nerve involvement) is not characteristic of these infections or their common sequelae.

Question 103: A 29-year-old man presents to the clinic complaining of fatigue and loss of sensation in his lower legs. The patient notes no history of trauma or chronic disease but states that he spends a lot of time outside and often encounters wild animals. On examination, the patient has multiple dark lesions over the skin of his face and back, as well as a decreased sensation of fine touch and vibration bilaterally in the lower extremities. What is the morphology of the etiologic agent causing this patient’s symptoms?

A. An acid-fast, intracellular bacillus (Correct Answer)
B. Reactivation of latent viral infection
C. Maltose-fermenting gram-negative diplococci
D. A spirochete transmitted via tick
E. Gram-positive, branching anaerobe

Explanation: ***An acid-fast, intracellular bacillus*** - The patient's symptoms (fatigue, loss of sensation in lower legs, dark skin lesions, exposure to wild animals, and decreased fine touch/vibration) are highly suggestive of **leprosy** (Hansen's disease). - Leprosy is caused by **_Mycobacterium leprae_**, which is an **acid-fast, obligate intracellular bacillus** that preferentially infects macrophages and Schwann cells, leading to nerve damage and skin lesions. *Reactivation of latent viral infection* - While viral infections can cause neurological symptoms, the presentation with specific **dark skin lesions** and the history of exposure to **wild animals** (potentially armadillos, a reservoir for _M. leprae_) point away from a common latent viral reactivation. - **Herpes zoster** (shingles) is a common reactivation of a latent viral infection (varicella-zoster virus), but it typically presents with a dermatomal rash and neuropathic pain rather than widespread dark lesions and bilateral sensory loss in the lower extremities. *Maltose-fermenting gram-negative diplococci* - **Maltose-fermenting gram-negative diplococci** describe **_Neisseria meningitidis_**, which causes meningitis. - This organism primarily causes symptoms related to meningitis (fever, headache, neck stiffness), and does not typically present with the chronic sensory loss or characteristic skin lesions seen in this patient. *A spirochete transmitted via tick* - A **spirochete transmitted via a tick** refers to **_Borrelia burgdorferi_**, the causative agent of **Lyme disease**. - Lyme disease presents with an **erythema migrans rash** (bull's-eye rash), joint pain, and neurological symptoms, but the skin lesions described here (multiple dark lesions) and the progressive sensory loss are not typical for Lyme disease. *Gram-positive, branching anaerobe* - A **gram-positive, branching anaerobe** describes **_Actinomyces_ species**, which cause **actinomycosis**. - Actinomycosis typically presents with chronic abscesses and sinus tracts, often affecting the head and neck, thorax, or abdomen, and does not cause the diffuse sensory loss or skin lesions described in this case.

Question 104: A 31-year-old female presents to the emergency room complaining of fever and difficulty breathing. She first noticed these symptoms 3 days prior to presentation. Her past medical history is notable for well-controlled asthma. She does not smoke and drinks alcohol socially. Upon further questioning, she reports that her urine is tea-colored when she wakes up but generally becomes more yellow and clear over the course of the day. Her temperature is 100.8°F (38.2°C), blood pressure is 135/90 mmHg, pulse is 115/min, and respirations are 20/min. Lung auscultation reveals rales at the right lung base. Laboratory analysis is shown below: Hemoglobin: 9.4 g/dL Hematocrit: 31% Leukocyte count: 3,700 cells/mm^3 with normal differential Platelet count: 110,000/mm^3 Reticulocyte count: 3% A chest radiograph reveals consolidation in the right lung base and the patient is given oral antibiotics. Which of the following processes is likely impaired in this patient?

A. Inactivation of C5 convertase (Correct Answer)
B. Inactivation of C3 convertase
C. Aminolevulinic acid metabolism
D. Nicotinamide dinucleotide phosphate metabolism
E. Erythrocyte cytoskeletal formation

Explanation: ***Inactivation of C5 convertase*** - This patient presents with **paroxysmal nocturnal hemoglobinuria (PNH)**, characterized by **tea-colored urine in the morning that clears during the day** (pathognomonic finding), **hemolytic anemia** (low hemoglobin/hematocrit, elevated reticulocyte count), and **pancytopenia** (leukopenia, thrombocytopenia). - PNH is caused by an acquired mutation in the **PIG-A gene** in hematopoietic stem cells, leading to deficiency of **glycosylphosphatidylinositol (GPI) anchors**. This results in loss of **CD55 (decay-accelerating factor, DAF)** and **CD59 (membrane inhibitor of reactive lysis, MIRL)** from cell surfaces. - **CD59** is the critical protein that **inactivates C5 convertase** and prevents formation of the **membrane attack complex (MAC)**. Without CD59, there is uncontrolled complement-mediated lysis of red blood cells, leading to intravascular hemolysis and hemoglobinuria. The nocturnal pattern occurs because mild acidosis during sleep enhances complement activation. - The pneumonia likely triggered increased complement activation, worsening the hemolysis. *Inactivation of C3 convertase* - **CD55 (DAF)** inactivates C3 convertase, and this is also impaired in PNH. - However, **CD59 deficiency and impaired C5 convertase inactivation** is the primary mechanism of intravascular hemolysis in PNH, as it directly prevents MAC formation. Both are impaired, but C5 convertase inactivation is more directly responsible for the clinical picture. *Nicotinamide dinucleotide phosphate metabolism* - This refers to **glucose-6-phosphate dehydrogenase (G6PD) deficiency**, which causes hemolytic anemia triggered by **oxidative stress** (infections, certain drugs, fava beans). - G6PD deficiency does NOT present with the **pathognomonic morning hemoglobinuria** seen in this patient, and would not cause **pancytopenia** (only affects RBCs). The blood smear would show bite cells and Heinz bodies, not mentioned here. *Aminolevulinic acid metabolism* - This pathway is involved in **heme synthesis**. Defects cause **porphyrias**, which present with neurological symptoms, photosensitivity, or abdominal pain—not hemolytic anemia with nocturnal hemoglobinuria. *Erythrocyte cytoskeletal formation* - Defects in cytoskeletal proteins (spectrin, ankyrin) cause **hereditary spherocytosis** or **elliptocytosis**. - These cause chronic hemolytic anemia but do NOT present with **nocturnal hemoglobinuria** or **pancytopenia**, and are hereditary rather than acquired.

Question 105: A 59-year-old woman comes to the emergency department because of abdominal pain and bloody diarrhea that began 12 hours ago. Three days ago, she ate undercooked chicken at a local restaurant. Blood cultures grow spiral and comma-shaped, oxidase-positive organisms at 42°C. This patient is at greatest risk for which of the following complications?

A. Toxic megacolon
B. Segmental myelin degeneration (Correct Answer)
C. Peyer patch necrosis
D. Seizures
E. Erythema nodosum

Explanation: ***Segmental myelin degeneration*** - The description of the organism (spiral/comma-shaped, oxidase-positive, growing at 42°C) combined with bloody diarrhea from undercooked chicken strongly suggests **_Campylobacter jejuni_** infection. - _Campylobacter jejuni_ infection is the most common antecedent infection for **Guillain-Barré syndrome (GBS)**, which is characterized by **segmental demyelination** of peripheral nerves. *Toxic megacolon* - This complication is more commonly associated with severe inflammatory bowel disease or infections like **_Clostridium difficile_** or **_Entamoeba histolytica_**. - While theoretically possible with any severe diarrheal illness, it is not the **greatest risk** specifically linked to _Campylobacter_ in this context. *Peyer patch necrosis* - **Peyer patch necrosis** is a characteristic complication seen in **typhoid fever**, caused by **_Salmonella Typhi_**, which would present differently (e.g., rose spots, bradycardia, step-wise fever). - _Campylobacter_ infection primarily causes inflammation of the intestinal mucosa rather than necrosis of Peyer patches. *Seizures* - Seizures are not a typical or common direct complication of _Campylobacter jejuni_ infection in adults, although severe electrolyte imbalances from any diarrhea could rarely precipitate them. - They are more commonly associated with direct CNS infections or severe systemic inflammatory responses from other pathogens. *Erythema nodosum* - **Erythema nodosum** is a non-specific inflammatory condition that can be associated with various infections, including streptococcal infections, tuberculosis, and some fungal diseases, as well as inflammatory bowel disease. - While it has been reported rarely with _Campylobacter_ infections, it is not the **most significant or common complication** compared to GBS.

Question 106: A 22-year-old woman comes to the urgent care clinic with sudden onset of severe vomiting. She had been at a picnic with her boyfriend a few hours earlier, enjoying barbecue, potato salad, and cake. Shortly thereafter, she began vomiting and has vomited 5 times in the last 3 hours. She has no prior history of symptoms. After a few hours of observation, her symptoms abate, and she is safely discharged home. Which of the following is the most likely cause of her vomiting?

A. Toxin ingestion from spore-forming organism
B. Toxin ingestion from non-spore-forming organism (Correct Answer)
C. Hepatitis
D. Gallstones
E. Viral infection

Explanation: ***Toxin ingestion from non-spore-forming organism*** - The rapid onset (within hours) and prompt resolution of severe vomiting following a meal **strongly suggest preformed bacterial toxins** in the food. - **Staphylococcus aureus** is the most common cause of such rapid-onset food poisoning, producing **preformed enterotoxins** in improperly stored foods (especially dairy products, mayonnaise-based salads, and cream-filled desserts). - Classic presentation: **1-6 hour incubation**, predominant **vomiting** (often severe), and **rapid resolution** within 24 hours. - The toxin is heat-stable and not destroyed by reheating food. *Toxin ingestion from spore-forming organism* - Spore-forming organisms include ***Clostridium perfringens*** and ***Bacillus cereus***. - ***C. perfringens***: Incubation **8-16 hours**, causes predominantly **diarrhea and abdominal cramps** rather than immediate severe vomiting. - ***B. cereus* (emetic form)**: Can cause rapid-onset vomiting (1-5 hours) but is **less common** than *S. aureus* in this clinical scenario. - The rapid onset and resolution pattern in this case is **most consistent with *S. aureus*** rather than spore-forming organisms. *Hepatitis* - **Acute hepatitis** typically presents with fatigue, nausea, vomiting, abdominal pain, and jaundice, but symptoms develop over **days to weeks**. - The **sudden onset** and **rapid resolution** within hours make acute hepatitis an unlikely cause. *Gallstones* - **Cholecystitis** or **biliary colic** due to gallstones can cause nausea and vomiting, typically associated with severe **right upper quadrant pain**, especially after fatty meals. - The lack of abdominal pain and quick resolution without intervention make gallstone-related issues less probable. *Viral infection* - **Viral gastroenteritis** (e.g., norovirus) typically has an incubation period of **12-48 hours**, making rapid onset within a few hours unlikely. - While it can cause sudden vomiting and diarrhea, symptoms usually persist for **24-72 hours**, not resolving within a few hours as described.

Question 107: A 55-year-old woman presents to her primary care physician for a worsening cough. She states that she has had a cough for 5 months. Over the past 2 weeks, the cough has become more frequent and produces yellow sputum. She has dyspnea on exertion at baseline, which she feels is also worsening. She denies fever, hemoptysis, or chest pain. She has chronic obstructive pulmonary disease and mild osteoarthritis. She uses inhaled ipratropium and takes ibuprofen as needed. She received the influenza vaccine 2 months ago. She smokes a half pack a day, and denies alcohol or recreational drug use. In addition to broad-spectrum antibiotics, which of the following is indicated?

A. Inhaled bronchodilators
B. Oxygen therapy
C. Chest physiotherapy
D. Systemic corticosteroids (Correct Answer)
E. Mucolytic agents

Explanation: ***Systemic corticosteroids*** - This patient is experiencing an **acute exacerbation of COPD**, indicated by worsening cough, increased sputum production, and increased dyspnea. Systemic corticosteroids are **anti-inflammatory agents** that help reduce airway inflammation and improve lung function during exacerbations. - Guidelines recommend a short course of oral corticosteroids (e.g., prednisone 40 mg daily for 5 days) in addition to broad-spectrum antibiotics for moderate to severe COPD exacerbations. *Inhaled bronchodilators* - The patient already uses **inhaled ipratropium**, an anticholinergic bronchodilator, for her COPD, and the question asks what else is indicated *in addition* to other treatments. While bronchodilators are fundamental for COPD management, the primary additional intervention for an exacerbation, beyond her baseline treatment and antibiotics, is systemic corticosteroids. - Although increasing the frequency or dosage of short-acting bronchodilators (e.g., albuterol) might be part of exacerbation management, it's not the *most critical additional treatment* compared to systemic corticosteroids which address the underlying inflammation. *Oxygen therapy* - Oxygen therapy is indicated for patients with **hypoxemia**, but the vignette does not provide information about her oxygen saturation or arterial blood gas results to suggest she is hypoxemic. - While it's a critical supportive treatment for severe exacerbations with hypoxemia, it's not universally indicated for every COPD exacerbation, especially in the absence of documented low oxygen levels. *Chest physiotherapy* - Chest physiotherapy techniques (e.g., percussion, postural drainage) are primarily used to help clear secretions in conditions like **cystic fibrosis** or **bronchiectasis**. - While it *can* assist with sputum clearance in some COPD patients, it's not a standard *first-line treatment* for an acute exacerbation, nor is it as universally indicated as systemic corticosteroids for reducing inflammation and symptoms. *Mucolytic agents* - Mucolytic agents (e.g., N-acetylcysteine) can help **thin mucus** and make it easier to clear, and may be considered in some patients with chronic productive cough. - However, they are not a primary treatment for an **acute COPD exacerbation** and are not as effective as systemic corticosteroids in rapidly resolving the inflammatory component of the exacerbation.

Question 108: Part of the success of the Streptococcus pyogenes bacterium lies in its ability to evade phagocytosis. Which of the following helps in this evasion?

A. Streptolysin S
B. Streptolysin O
C. Streptokinase
D. M protein (Correct Answer)
E. Pyrogenic toxin

Explanation: ***M protein*** - The **M protein** is a major virulence factor of *Streptococcus pyogenes* that **inhibits phagocytosis** by binding to factor H, a host complement regulatory protein, preventing C3b deposition. - It also helps the bacterium adhere to host cells and resist killing by neutrophils. *Streptolysin S* - **Streptolysin S** is a **hemolysin** that causes beta-hemolysis on blood agar and contributes to tissue damage by lysing cells. - While contributing to virulence, its primary role is not direct inhibition of phagocytosis but rather cell lysis. *Streptolysin O* - **Streptolysin O (SLO)** is another **hemolysin** that produces pore-forming toxins, leading to cell lysis and tissue destruction. - It is highly antigenic and often used as a diagnostic marker (ASO titer) for past *S. pyogenes* infections, but it does not directly prevent phagocytosis. *Streptokinase* - **Streptokinase** is an enzyme that activates plasminogen, leading to the breakdown of fibrin clots, which helps in the **spread of infection** within tissues. - Its main function is not to evade phagocytosis but rather to facilitate invasion by dissolving blood clots that would typically wall off the infection. *Pyrogenic toxin* - **Pyrogenic toxins** (also known as erythrogenic toxins) are superantigens that cause symptoms like fever and rash (e.g., in scarlet fever) by stimulating a massive, non-specific T-cell activation. - These toxins contribute to the systemic manifestations of infection but do not directly interfere with the process of phagocytosis.

Question 109: A 55-year-old woman comes to the physician because of fevers for 2 weeks. She works as a nurse and recently returned from a charity work trip to India, where she worked in a medically-underserved rural community. A tuberculin skin test 3 months ago prior to her trip showed an induration of 3 mm. Physical examination is unremarkable. An x-ray of the chest shows right-sided hilar lymphadenopathy. A sputum culture shows acid-fast bacilli. Which of the following immunologic processes most likely occurred first?

A. Production of interferon-gamma by T-helper cells
B. Migration of T-helper cells to the lungs
C. Replication of bacteria within alveolar macrophages (Correct Answer)
D. Formation of a nodular tubercle in the lung
E. Transportation of bacterial peptides to regional lymph nodes

Explanation: ***Replication of bacteria within alveolar macrophages*** - After initial infection, **Mycobacterium tuberculosis** is phagocytosed by **alveolar macrophages** in the lungs, where it **replicates unimpeded** for about 2–4 weeks before the adaptive immune response is fully mounted. - This phase of unchecked bacterial growth precedes the immune system's attempt to contain the infection, making it the first significant immunologic event. *Transportation of bacterial peptides to regional lymph nodes* - This process involves **antigen-presenting cells** (APCs) — typically macrophages or dendritic cells — migrating from the lungs to regional lymph nodes to present bacterial antigens to T cells. - This step occurs *after* the initial bacterial replication and phagocytosis but *before* a robust T-cell mediated immune response develops, as T-cells need to be activated in the lymph nodes. *Formation of a nodular tubercle in the lung* - The **tubercle** (granuloma) is a hallmark of tuberculosis, representing the body's attempt to contain the infection. - Its formation is a complex process involving activated macrophages, T cells, and fibroblasts, and it occurs *after* the initial bacterial replication and the subsequent immune cell activation and recruitment. *Migration of T-helper cells to the lungs* - **T-helper cells** migrate to the lungs only after they have been **activated** in the regional lymph nodes by antigen-presenting cells. - This migration is crucial for orchestrating the immune response and containing the infection but happens *after* initial bacterial proliferation and antigen presentation. *Production of interferon-gamma by T-helper cells* - **Interferon-gamma** (IFN-$\gamma$) is a key cytokine produced by activated T-helper cells (Th1 cells) that activates macrophages to become more effective at killing intracellular bacteria. - This production signifies a mature adaptive immune response and occurs *after* T-helper cell activation in the lymph nodes and subsequent migration to the infected site.

Question 110: A 78-year-old man presented to his primary physician with a 3-month history of weight loss, fever, fatigue, night sweats, and cough. He is a former smoker. A recent HIV test was negative. A CT scan of the chest reveals a 3 cm lesion in the lower lobe of the left lung and calcification around the left lung hilus. A sputum smear was positive for acid fast organisms. These findings are most consistent with which of the following:

A. Adenocarcinoma
B. Secondary tuberculosis (Correct Answer)
C. Miliary tuberculosis
D. Primary tuberculosis
E. Coccidioidomycosis infection

Explanation: ***Correct: Secondary tuberculosis*** - The **hilar calcification** indicates **prior primary TB infection** that was contained, forming a healed Ghon complex, which is now **reactivating**. - The **3 cm active lesion** with positive **acid-fast organisms** in sputum represents **reactivation tuberculosis** (secondary TB) occurring in a patient with previous exposure. - In elderly patients (78 years old), **immunosenescence** increases risk of reactivation from latent foci, even in non-apical locations. - Constitutional symptoms (weight loss, fever, night sweats) and positive sputum smear confirm active TB disease. *Incorrect: Primary tuberculosis* - Primary TB represents the **initial infection** in a previously unexposed individual, typically resulting in a Ghon complex (calcified primary focus + lymph node). - This patient has **evidence of prior TB** (hilar calcification), making this a reactivation, not primary infection. - Primary TB usually presents with **lymphadenopathy and pleural effusion** rather than cavitary lesions and positive sputum. *Incorrect: Coccidioidomycosis infection* - While coccidioidomycosis can cause lung lesions and constitutional symptoms, it is a **fungal infection** and would not present with **acid-fast organisms** in sputum. - It is geographically localized to the **southwestern United States** and parts of Central/South America. - Hilar calcification from prior TB would be coincidental, not related to current coccidioidal infection. *Incorrect: Adenocarcinoma* - Adenocarcinoma can cause lung lesions, weight loss, and fatigue in a former smoker, but it is a **malignancy**. - Sputum would show **malignant cells**, not **acid-fast organisms**. - The positive AFB smear definitively indicates mycobacterial infection, not cancer. *Incorrect: Miliary tuberculosis* - **Miliary TB** involves hematogenous dissemination resulting in **diffuse bilateral micronodular lesions** (1-3 mm) throughout both lungs. - This patient has a **single 3 cm lesion**, which is inconsistent with the military (millet seed-like) pattern. - Miliary TB typically occurs in severely immunocompromised patients; this patient is HIV-negative.

Question 111: A 70-year-old man with loose stools over the last 24 hours, accompanied by abdominal pain, cramps, nausea, and anorexia, was hospitalized. Previously, the man was diagnosed with a lung abscess and was treated with clindamycin for 5 days. Past medical history was significant for non-erosive antral gastritis and hypertension. He takes esomeprazole and losartan. Despite the respiratory improvement, fevers and leukocytosis persisted. Which of the following pathogenic mechanisms would you expect to find in this patient?

A. Glucosylation of Rho family GTPases (Correct Answer)
B. Inactivation of elongation factor EF-2
C. Inactivation of the 60S ribosome subunit
D. Cell membrane degradation by lecithinase
E. ADP-ribosylation of Gs-alpha subunit of G-protein coupled receptors

Explanation: ***Glucosylation of Rho family GTPases*** - The clinical presentation (clindamycin use, loose stools, abdominal pain, fever, leukocytosis) strongly suggests **_Clostridioides difficile_ infection (CDI)**. The **_C. difficile_ toxins A and B** are glucosyltransferases that modify and inactivate Rho family GTPases. - **Inactivation of Rho GTPases** leads to disruption of the **cytoskeleton**, loss of tight junctions between enterocytes, and ultimately causes **cell death and colonic inflammation**, resulting in pseudomembranous colitis. *Inactivation of elongation factor EF-2* - This is the mechanism of action of **diphtheria toxin** (produced by **_Corynebacterium diphtheriae_**) and **_Pseudomonas aeruginosa_ exotoxin A**. - These toxins **ADP-ribosylate elongation factor 2 (EF-2)**, inhibiting protein synthesis and leading to cell death. This does not align with the patient's symptoms or antibiotic history. *Inactivation of the 60S ribosome subunit* - This mechanism is associated with **Shiga toxin** (produced by **_Shigella dysenteriae_** and **enterohemorrhagic _E. coli_ (EHEC)**) and **ricin toxin**. - These toxins enzymatically remove an adenine residue from the 28S rRNA of the 60S ribosomal subunit, thereby **halting protein synthesis** and causing cell damage. *Cell membrane degradation by lecithinase* - **Lecithinase (alpha-toxin)** produced by **_Clostridium perfringens_** is a phospholipase that degrades **lecithin** in cell membranes. - This leads to **hemolysis, myonecrosis, and tissue destruction** characteristic of gas gangrene, which is not consistent with the patient's diarrheal illness. *ADP-ribosylation of Gs-alpha subunit of G-protein coupled receptors* - This is the mechanism of action of **cholera toxin** (produced by **_Vibrio cholerae_**) and **heat-labile enterotoxin (LT)** of **enterotoxigenic _E. coli_ (ETEC)**. - **ADP-ribosylation of Gs-alpha subunit** permanently activates adenylate cyclase, leading to increased intracellular **cAMP**, which causes excessive **secretion of water and electrolytes** into the gut lumen, resulting in watery diarrhea, but without significant inflammation as seen in the patient.

Question 112: A previously healthy 25-year-old male comes to his primary care physician with a painless solitary lesion on his penis that developed 4 days ago. He has not experienced anything like this before. He is currently sexually active with multiple partners and uses condoms inconsistently. His temperature is 37.0°C (98.7°F), pulse is 67/min, respirations are 17/min, and blood pressure is 110/70 mm Hg. Genitourinary examination shows a shallow, nontender, firm ulcer with a smooth base along the shaft of the penis. There is nontender inguinal adenopathy bilaterally. Which of the following is the most appropriate next step to confirm the diagnosis?

A. Rapid plasma reagin
B. Urine polymerase chain reaction
C. Fluorescent treponemal antibody absorption test
D. Swab culture
E. Dark-field microscopy (Correct Answer)

Explanation: ***Dark-field microscopy*** - The patient's presentation with a **painless, firm, shallow ulcer** (chancre) on the penis and **bilateral nontender inguinal adenopathy**, in the context of high-risk sexual behavior, is highly suggestive of **primary syphilis**. - **Dark-field microscopy** of exudate from the chancre allows for direct visualization of motile *Treponema pallidum* spirochetes and is the definitive method for confirming primary syphilis, especially before serological tests become positive. *Rapid plasma reagin* - **RPR is a nontreponemal serological test** used for screening syphilis. It typically becomes reactive 1-3 weeks after the appearance of a chancre. - Given that the lesion developed only 4 days ago, the RPR might still be **negative due to the lag phase** before antibody production. *Urine polymerase chain reaction* - A **urine PCR** is primarily used to detect nucleic acids of infectious agents, commonly for conditions like chlamydia or gonorrhea. - It is **not the standard or most accurate method** for diagnosing syphilis, which is caused by a spirochete and typically diagnosed by direct visualization or serology. *Fluorescent treponemal antibody absorption test* - The **FTA-ABS is a treponemal-specific serological test** that usually becomes reactive earlier than non-treponemal tests (like RPR), but still typically weeks after infection. - While sensitive, it is generally used as a **confirmatory test** for positive nontreponemal results or when clinical suspicion is high and nontreponemal tests are initially negative. It is not a direct detection method. *Swab culture* - **Swab culture** is used to grow bacteria for identification and susceptibility testing. - *Treponema pallidum*, the causative agent of syphilis, **cannot be cultured on artificial media**, making swab culture an inappropriate diagnostic method for syphilis.

Question 113: A 6-year-old boy presents with fever, sore throat, hoarseness, and neck enlargement. The symptoms started 3 days ago and progressed gradually with an elevated temperature and swollen lymph nodes. His family immigrated recently from Honduras. He was born via spontaneous vaginal delivery at 39 weeks after an uneventful gestational period and he is now on a catch-up vaccination schedule. He lives with several family members, including his parents, in a small apartment. No one in the apartment smokes tobacco. On presentation, the patient’s blood pressure is 110/75 mm Hg, heart rate is 103/min, respiratory rate is 20/min, and temperature is 39.4°C (102.9°F). On physical examination, the child is acrocyanotic and somnolent. There is widespread cervical edema and enlargement of the cervical lymph nodes. The tonsils are covered with a gray, thick membrane which spreads beyond the tonsillar bed and reveals bleeding, erythematous mucosa with gentle scraping. The lungs are clear to auscultation. Which of the following is the target of the virulence factor produced by the pathologic organism infecting this child?

A. Desmoglein
B. SNAP-25
C. ADP-ribosylation factor 6
D. Eukaryotic elongation factor-2 (eEF-2) (Correct Answer)
E. RNA polymerase II

Explanation: ***Eukaryotic elongation factor-2 (eEF-2)*** - The clinical presentation (fever, sore throat, hoarseness, neck enlargement, and a **gray, thick membrane** on tonsils that bleeds on scraping, accompanied by **acrocyanosis and somnolence**) is highly suggestive of **diphtheria**, caused by *Corynebacterium diphtheriae*. - The **diphtheria toxin** produced by *C. diphtheriae* is an **exotoxin** that acts by **ADP-ribosylating and inactivating eukaryotic elongation factor-2 (eEF-2)**, thereby inhibiting protein synthesis and leading to cell death. *Desmoglein* - **Desmoglein** is a component of **desmosomes** targeted by **autoantibodies** in **pemphigus vulgaris**, a blistering skin disease. - This is not related to the mechanism of action of the diphtheria toxin. *SNAP-25* - **SNAP-25** is a protein involved in the release of **neurotransmitters** at the **neuromuscular junction**. - It is cleaved by **botulinum toxin** (produced by *Clostridium botulinum*), leading to flaccid paralysis; it is not the target of diphtheria toxin. *ADP-ribosylation factor 6* - **ADP-ribosylation factor 6 (ARF6)** is a small GTPase involved in regulating vesicular trafficking and actin dynamics. - While other bacterial toxins, such as **cholera toxin** and **pertussis toxin**, also cause ADP-ribosylation, their targets and clinical effects differ significantly from diphtheria toxin. *RNA polymerase II* - **RNA polymerase II** is responsible for transcribing **mRNA** in eukaryotes. - Some toxins, like **alpha-amanitin** from *Amanita phalloides* mushrooms, inhibit RNA polymerase II, but this is not the target of the diphtheria toxin.

Question 114: While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

A. Quellung reaction (Correct Answer)
B. Hemolytic reaction when grown on sheep blood agar
C. Bile solubility
D. Optochin sensitivity
E. Motility

Explanation: ***Quellung reaction*** - The **Quellung reaction** tests for the presence of the **polysaccharide capsule**, which is the primary virulence factor of *S. pneumoniae*. - An **avirulent strain** that cannot cause disease would most likely lack the capsule and show a **negative Quellung reaction** (no capsular swelling), deviating from the **positive reaction** seen in typical encapsulated pathogenic strains. - The capsule enables *S. pneumoniae* to evade phagocytosis and complement-mediated killing, which is essential for establishing infection in the lungs. *Hemolytic reaction when grown on sheep blood agar* - Both virulent and avirulent strains of *S. pneumoniae* typically exhibit **alpha-hemolysis** (partial hemolysis, producing a greenish discoloration) on sheep blood agar due to the production of pneumolysin. - This characteristic does not differentiate between pathogenic and non-pathogenic strains in terms of disease-causing ability. *Bile solubility* - *S. pneumoniae* is characteristically **bile-soluble** due to the presence of autolysin enzymes that are activated by bile salts, leading to cellular lysis. - This property is a **species characteristic** present in both virulent and avirulent strains, thus it would not explain the inability to cause disease. *Optochin sensitivity* - *S. pneumoniae* is universally **sensitive to optochin**, a chemical agent that inhibits its growth and is used for laboratory identification. - This characteristic is used for **species identification** but does not correlate with strain virulence or disease-causing ability. *Motility* - *Streptococcus pneumoniae* is a **non-motile** bacterium; it lacks flagella. - This characteristic is consistent across all strains and is not a virulence factor for this species.

Question 115: A 2-year-old boy is brought to the emergency department because of fever, cough, and ear pain over the past 2 days. He has had recurrent respiratory tract infections and several episodes of giardiasis and viral gastroenteritis since he was 6 months of age. Examination shows decreased breath sounds over both lung fields and bilateral purulent otorrhea. His palatine tonsils and adenoids are hypoplastic. Quantitative flow cytometry of his blood shows decreased levels of cells that express CD19, CD20, and CD21. Which of the following is the most likely cause of this patient's condition?

A. Mutation in NADPH oxidase gene
B. Microdeletion on the long arm of chromosome 22
C. Mutation in WAS gene
D. Mutation in tyrosine kinase gene (Correct Answer)
E. Defect in beta-2 integrin

Explanation: ***Mutation in tyrosine kinase gene*** - The patient's history of **recurrent bacterial infections** (respiratory tract infections, otorrhea), **giardiasis**, and **viral gastroenteritis**, combined with **hypoplastic tonsils and adenoids**, points to a **B-cell primary immunodeficiency**. - **Decreased levels of CD19+, CD20+, and CD21+ cells** on flow cytometry confirm a B-cell deficiency, making a mutation in the **tyrosine kinase gene (Bruton's tyrosine kinase, BTK)** the most likely cause, leading to **X-linked agammaglobulinemia (XLA)**. *Mutation in NADPH oxidase gene* - This mutation causes **chronic granulomatous disease (CGD)**, characterized by recurrent bacterial and fungal infections due to impaired phagocyte oxidative burst. - Patients with CGD typically have normal numbers of B cells (CD19+, CD20+, CD21+ cells); the clinical presentation would not include hypoplastic tonsils or repeated Giardia infection as a primary hallmark. *Microdeletion on the long arm of chromosome 22* - This describes **DiGeorge syndrome**, which involves T-cell deficiency due to thymic hypoplasia, along with cardiac defects and hypocalcemia. - While it can manifest with recurrent infections and sometimes lymphoid hypoplasia, the primary immune defect is in T cells, and B cell numbers would be normal unless severe secondary lymphoid tissue defects are present. *Mutation in WAS gene* - This causes **Wiskott-Aldrich syndrome (WAS)**, characterized by the triad of **thrombocytopenia** (small platelets), **eczema**, and **recurrent infections**. - WAS primarily affects T cells and platelet function; it would not typically present with hypoplastic tonsils or a primary B-cell count deficiency as the main diagnostic feature. *Defect in beta-2 integrin* - A defect in beta-2 integrin causes **leukocyte adhesion deficiency (LAD)**, characterized by recurrent bacterial infections, **impaired wound healing**, and **delayed umbilical cord separation**. - Patients with LAD have normal B cell numbers and do not typically present with hypoplastic lymphoid organs or a predisposition to giardiasis in the same manner as XLA.

Question 116: A 46-year-old woman presents to the emergency department complaining of bloody diarrhea, fatigue, and confusion. A few days earlier she went to a fast-food restaurant for a college reunion party. Her friends are experiencing similar symptoms. Laboratory tests show anemia, thrombocytopenia, and uremia. Lactate dehydrogenase (LDH) is raised while haptoglobin is decreased. Peripheral blood smears show fragmented red blood cells (RBCs). Coombs tests are negative. Which of the following is the responsible organism?

A. Entamoeba histolytica
B. Salmonella
C. Shigella
D. E. coli (Correct Answer)
E. Campylobacter jejuni

Explanation: ***E. coli*** - The combination of **bloody diarrhea**, acute renal failure (**uremia**), **thrombocytopenia**, and **microangiopathic hemolytic anemia** (fragmented RBCs, elevated LDH, decreased haptoglobin) after consuming fast food is characteristic of **Hemolytic-Uremic Syndrome (HUS)**. - **Shiga toxin-producing E. coli (STEC)**, particularly **E. coli O157:H7**, is the most common cause of HUS acquired from contaminated food, especially undercooked ground beef. *Entamoeba histolytica* - Causes **amebic dysentery** with bloody stools but typically does not lead to **HUS** with microangiopathic hemolytic anemia and renal failure. - More common in areas with poor sanitation and transmitted via the **fecal-oral route**, often presenting with liver abscesses. *Salmonella* - Can cause **bloody diarrhea** (typhoid fever, non-typhoidal salmonellosis) and sometimes bacteremia but is not typically associated with **HUS**. - While it can be acquired from contaminated food, the constellation of symptoms points away from Salmonella as the primary cause. *Shigella* - Causes **shigellosis**, a dysentery characterized by **bloody diarrhea**, and can produce **Shiga toxin**. - While it can occasionally cause **HUS**, especially in children, E. coli O157:H7 is a more frequent cause of foodborne HUS cases. *Campylobacter jejuni* - A common cause of **bacterial gastroenteritis** with **bloody diarrhea**, which can be acquired from contaminated food (e.g., undercooked poultry). - While it is associated with **Guillain-Barré syndrome**, it is rarely a cause of **HUS**.

Question 117: A 28-year-old man presents to the clinic complaining of chronic joint pain and fatigue for the past 2 months. The patient states that he usually has pain in one of his joints that resolve but then seems to move to another joint. The patient notes no history of trauma but states that he has experienced some subjective fevers over that time. He works as a logger and notes that he's heard that people have also had these symptoms in the past, but that he does not know anyone who is currently experiencing them. What is the most likely etiologic agent of this patient's disease?

A. A gram-positive, spore-forming rod
B. ssDNA virus of the Parvoviridae family
C. A gram-positive cocci in chains
D. A spirochete (Correct Answer)
E. A gram-negative diplococci

Explanation: ***A spirochete*** - The migrating joint pain (**migratory polyarthritis**), fatigue, and subjective fevers in a patient who works outdoors (logger) are highly suggestive of **Lyme disease**. - Lyme disease is caused by **_Borrelia burgdorferi_**, which is a **spirochete** transmitted by ticks. *A gram-positive, spore-forming rod* - This description typically refers to bacteria like **_Clostridium_** or **_Bacillus_** species. - These organisms are generally associated with conditions like **tetanus**, **botulism**, or **anthrax**, which do not match the migratory joint pain and fatigue described. *ssDNA virus of the Parvoviridae family* - The most common human pathogen in this family is **Parvovirus B19**, which causes **fifth disease** (erythema infectiosum). - While Parvovirus B19 can cause **arthralgia** and **arthritis**, especially in adults, it typically presents with a characteristic **slapped-cheek rash** and is less commonly associated with a prolonged, migratory joint pain pattern in this context. *A gram-positive cocci in chains* - This describes organisms like **_Streptococcus pyogenes_**, which can cause **rheumatic fever** leading to migratory polyarthritis. - However, rheumatic fever typically follows a **streptococcal pharyngitis** and has other characteristic features like carditis or chorea not mentioned here. *A gram-negative diplococci* - This describes organisms like **_Neisseria gonorrhoeae_** or **_Neisseria meningitidis_**. - **Disseminated gonococcal infection** can cause migratory polyarthralgia, but it is typically associated with a history of sexually transmitted infection and often presents with tenosynovitis or skin lesions, which are not mentioned.

Question 118: An 11-month-old boy presents to his pediatrician with severe wheezing, cough, and fever of 38.0°C (101.0°F). Past medical history is notable for chronic diarrhea since birth, as well as multiple pyogenic infections. The mother received prenatal care, and delivery was uneventful. Both parents, as well as the child, are HIV-negative. Upon further investigation, the child is discovered to have Pneumocystis jirovecii pneumonia, and the appropriate treatment is begun. Additionally, a full immunologic check-up is ordered. Which of the following profiles is most likely to be observed in this patient?

A. Increased IgE
B. Decreased IgM and increased IgE and IgA
C. Increased IgM and decreased IgA, IgG, and IgE (Correct Answer)
D. Increased IgE and decreased IgA and IgM
E. Decreased IgE, IgM, IgA, and IgG

Explanation: ***Increased IgM and decreased IgA, IgG, and IgE*** - The clinical picture of **Pneumocystis jirovecii pneumonia (PCP)**, chronic diarrhea, and recurrent pyogenic infections in an HIV-negative infant is highly suggestive of **Hyper-IgM Syndrome**. - In Hyper-IgM Syndrome, there's a defect in the **CD40-CD40L interaction**, preventing B cells from undergoing class switching, leading to normal or elevated IgM but deficient IgG, IgA, and IgE. *Increased IgE* - While IgE can be elevated in certain immunodeficiencies like **Job syndrome (Hyper-IgE syndrome)**, the primary absence of IgG and IgA, and the presence of severe pyogenic infections and PCP, point away from isolated IgE elevation as the sole or primary defect. - Job syndrome presents with distinct features like **eczema**, characteristic facial features, and cold abscesses, which are not mentioned here. *Decreased IgM and increased IgE and IgA* - This profile does not align with a recognized primary immunodeficiency that would cause PCP and recurrent pyogenic infections. - Reduced IgM typically indicates a problem with initial **antibody production**, which would then affect other immunoglobulins, but not necessarily result in increased IgE and IgA. *Increased IgE and decreased IgA and IgM* - While IgA and IgM might be low in some conditions, an isolated elevation in IgE with combined IgA and IgM deficiency does not fit the characteristic presentation of recurrent pyogenic infections and PCP seen in this patient. - This profile is not typical of **Hyper-IgM syndrome**, where IgM is elevated or normal. *Decreased IgE, IgM, IgA, and IgG* - This represents **severe combined immunodeficiency (SCID)** or **common variable immunodeficiency (CVID)**. While SCID can present with PCP, it usually manifests earlier and with more profound immune dysfunction. - CVID typically presents later in childhood or adulthood, and while it involves low IgG, IgA, and IgM, the specific presentation with PCP and elevated IgM is more characteristic of Hyper-IgM syndrome.

Question 119: An 8-year-old female presents to her pediatrician with nasal congestion. Her mother reports that the patient has had nasal congestion and nighttime cough for almost two weeks. The patient’s 3-year-old brother had similar symptoms that began around the same time and have since resolved. The patient initially seemed to be improving, but four days ago she began developing worsening nasal discharge and fever to 102.6°F (39.2°C) at home. Her mother denies any change in appetite. The patient denies sore throat, ear pain, and headache. She is otherwise healthy. In the office, her temperature is 102.2°F (39.0°C), blood pressure is 96/71 mmHg, pulse is 128/min, and respirations are 18/min. On physical exam, the nasal turbinates are edematous and erythematous. She has a dry cough. Purulent mucous can be visualized dripping from the posterior nasopharynx. Her maxillary sinuses are tender to palpation. Which of the following organisms is most likely to be causing this patient’s current condition?

A. Moraxella catarrhalis (Correct Answer)
B. Streptococcus pyogenes
C. Adenovirus
D. Streptococcus pneumoniae
E. Haemophilus influenzae

Explanation: ***Moraxella catarrhalis*** - This patient presents with **acute bacterial rhinosinusitis (ABRS)**, characterized by worsening nasal congestion, purulent discharge, high fever, and maxillary sinus tenderness after initial improvement from a viral illness (the classic **"double sickening"** pattern). - The three most common bacterial causes of ABRS in children are *Streptococcus pneumoniae* (most common, 30-40%), *Haemophilus influenzae* (20-30%), and ***Moraxella catarrhalis*** (10-20%). - **Given the options provided, *Moraxella catarrhalis* is the correct answer** as it is a well-established cause of pediatric ABRS, particularly in children under 10 years of age, and is the only common ABRS pathogen listed among the choices. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is associated with **nosocomial infections**, **chronic sinusitis in cystic fibrosis patients**, immunocompromised states, or malignant otitis externa. - It is not a typical cause of acute community-acquired bacterial rhinosinusitis in healthy, immunocompetent children. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) primarily causes **pharyngitis** with sore throat, tonsillar exudates, and cervical lymphadenopathy—none of which are present in this patient. - **Note:** This should not be confused with ***Streptococcus pneumoniae***, which is actually the **most common cause** of bacterial rhinosinusitis in children but is not listed as an option in this question. - *S. pyogenes* is not a recognized common cause of acute bacterial rhinosinusitis. *Adenovirus* - Adenovirus is a common viral cause of **upper respiratory tract infections** and likely caused this patient's initial illness. - The **biphasic course** with worsening symptoms after day 10, persistent high fever (>102°F), and purulent discharge indicate **bacterial superinfection**, not ongoing viral illness. - Viral URIs typically improve within 7-10 days without this pattern of clinical deterioration. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause sinusitis, it is **not a common cause** of acute community-acquired bacterial rhinosinusitis in otherwise healthy children. - *S. aureus* sinusitis is more commonly associated with **chronic sinusitis**, **nosocomial infections**, **post-surgical infections**, or complications such as orbital or intracranial extension.

Question 120: A 6-day-old boy is brought to the emergency room with a fever. He was born to a G1P1 mother at 39 weeks gestation via vaginal delivery. The mother underwent all appropriate prenatal care and was discharged from the hospital 1 day after birth. The boy has notable skin erythema around the anus with some serosanguinous fluid. The umbilical stump is present. The patient is discharged from the emergency room with antibiotics. He returns to the emergency room at 32 days of age and his mother reports that he has been clutching his left ear. The left tympanic membrane appears inflamed and swollen. The umbilical stump is still attached and is indurated, erythematous, and swollen. The boy's temperature is 99°F (37.2°C), blood pressure is 100/60 mmHg, pulse is 130/min, and respirations are 20/min. A complete blood count is shown below: Hemoglobin: 14.0 g/dL Hematocrit: 42% Leukocyte count: 16,000/mm^3 with normal differential Platelet count: 190,000/mm^3 A deficiency in which of the following compounds is most likely the cause of this patient's condition?

A. Immunoglobulin A
B. Tapasin
C. NADPH oxidase
D. IL-12 receptor
E. CD18 (Correct Answer)

Explanation: ***CD18*** - The patient's recurrent infections, particularly the persistent umbilical stump infection with erythema and induration and the severe skin infection (perianal erythema with serosanguinous fluid), suggest a defect in **leukocyte extravasation** and **phagocytosis**. - **CD18 deficiency** causes **Leukocyte Adhesion Deficiency Type 1 (LAD-1)**, characterized by recurrent bacterial infections, impaired wound healing, **omphalitis** (delayed umbilical cord separation), and a striking leukocytosis due to the inability of neutrophils to exit the vasculature. The elevated leukocyte count (16,000/mm^3) with normal differential despite infection further supports LAD-1. *Immunoglobulin A* - **IgA deficiency** is typically associated with recurrent **mucosal infections** (respiratory, gastrointestinal), **allergies**, and **autoimmune diseases**. - It does not explain the persistent umbilical stump, severe skin infections, or the elevated neutrophil count typical for this patient. *Tapasin* - **Tapasin** is involved in **MHC class I assembly** and peptide loading; its deficiency leads to a rare immunodeficiency affecting **cytotoxic T-cell responses** to intracellular pathogens. - While it can manifest as recurrent viral infections, it does not explain the recurrent bacterial skin infections, omphalitis, or persistent leukocytosis seen in this patient. *NADPH oxidase* - Deficiency in **NADPH oxidase** causes **Chronic Granulomatous Disease (CGD)**, characterized by recurrent infections with **catalase-positive organisms** and **granuloma formation**. - While CGD can present with recurrent bacterial infections, it does not typically cause delayed umbilical cord separation or the pronounced leukocytosis observed in this case. *IL-12 receptor* - Deficiency in the **IL-12 receptor** leads to impaired **Th1 cell responses** and disseminated infections, particularly with **atypical mycobacteria** and **intracellular bacteria** like *Salmonella*. - This deficiency does not align with the severe skin and umbilical stump infections or the specific laboratory findings of this patient.

Question 121: A medical technician is trying to isolate a pathogen from the sputum sample of a patient. The sample is heat fixed to a slide then covered with carbol fuchsin stain and heated again. After washing off the stain with clean water, the slide is covered with 1% sulfuric acid for decolorization. The sample is rinsed again and stained with methylene blue. Microscopic examination shows numerous red, branching filamentous organisms. Which of the following is the most likely isolated pathogen?

A. Cryptococcus neoformans
B. Tropheryma whipplei
C. Nocardia asteroides (Correct Answer)
D. Rickettsia rickettsii
E. Staphylococcus aureus

Explanation: ***Nocardia asteroides*** - The described staining procedure is a **modified acid-fast stain**, indicated by the use of **carbol fuchsin**, heating, and decolorization with **weak acid (1% sulfuric acid)**, followed by a counterstain with methylene blue. - **Nocardia species** are **weakly acid-fast bacteria** that resist decolorization with weak acids (1-3% sulfuric acid), appearing as **red, branching filamentous organisms** under this staining method. - The **modified acid-fast stain** uses weaker decolorizing agents compared to the standard Ziehl-Neelsen stain, making it suitable for detecting weakly acid-fast organisms like Nocardia. - Nocardia are aerobic actinomycetes commonly found in soil and can cause pulmonary infections, especially in immunocompromised patients. *Cryptococcus neoformans* - This is a **yeast** that is typically identified using an **India ink stain** to visualize its polysaccharide capsule, or through fungal stains like Gomori methenamine silver (GMS). - It would not appear as acid-fast red branching filaments with the described technique. *Tropheryma whipplei* - This bacterium is typically identified by **periodic acid-Schiff (PAS) stain** in tissue biopsies, which highlights its cell wall glycoproteins (appears magenta). - It is not acid-fast and would not retain the carbol fuchsin after acid decolorization. *Rickettsia rickettsii* - This is an **obligate intracellular bacterium** that is difficult to culture and is often diagnosed by **serological tests** or **immunohistochemistry** on skin biopsy specimens. - It is not acid-fast and would not be detected by this staining technique. *Staphylococcus aureus* - This is a **Gram-positive coccus** that would be stained **purple** by a Gram stain as it retains crystal violet. - It is not acid-fast and would be completely decolorized by sulfuric acid in the described procedure, appearing blue (counterstain color) rather than red.

Question 122: A 42-year-old woman comes to the emergency department because of worsening severe pain, swelling, and stiffness in her right knee for the past 2 days. She recently started running 2 miles, 3 times a week in an attempt to lose weight. She has type 2 diabetes mellitus and osteoporosis. Her mother has rheumatoid arthritis. She drinks one to two glasses of wine daily. She is sexually active with multiple partners and uses condoms inconsistently. Current medications include metformin and alendronate. She is 161 cm (5 ft 3 in) tall and weighs 74 kg (163 lb); BMI is 29 kg/m2. Her temperature is 38.3°C (100.9°F), pulse is 74/min, and blood pressure is 115/76 mm Hg. She appears to be in discomfort and has trouble putting weight on the affected knee. Physical examination shows a 2-cm, painless ulcer on the plantar surface of the right toe. The right knee is swollen and tender to palpation. Arthrocentesis of the right knee with synovial fluid analysis shows a cell count of 55,000 WBC/μL with 77% polymorphonuclear (PMN) cells. Which of the following is the most likely underlying cause of this patient's presenting condition?

A. Autoimmune response to bacterial infection
B. Hematogenous spread of infection (Correct Answer)
C. Occult meniscal tear
D. Intra-articular deposition of urate crystals
E. Direct inoculation of infectious agent

Explanation: ***Hematogenous spread of infection*** - The patient has a **diabetic foot ulcer**, which serves as a portal of entry for bacteria, and **poor glycemic control** (implied by type 2 diabetes) increases susceptibility to infections. - The **synovial fluid analysis** with a very high **WBC count (55,000 WBC/μL)** and **predominance of PMN cells (77%)**, along with fever and acute severe pain in a single joint, is highly suggestive of **septic arthritis**, which can occur via hematogenous spread from a distant infection site like the foot ulcer. *Autoimmune response to bacterial infection* - This describes **reactive arthritis**, which usually presents with sterile synovitis occurring days to weeks after an infection, commonly genitourinary or gastrointestinal. - While the patient's sexual history could be a risk factor for such infections, the **high synovial fluid WBC count with PMN predominance** and fever point more directly to active bacterial infection within the joint rather than a sterile autoimmune reaction. *Occult meniscal tear* - A meniscal tear typically causes mechanical pain, locking, or clicking, and while it can lead to swelling, it rarely presents with **fever** and such a **high inflammatory synovial fluid profile**. - The patient's recent running could contribute to a tear, but the systemic signs of infection and significant synovial inflammation make this diagnosis less likely. *Intra-articular deposition of urate crystals* - This describes **gout**, which presents with acute, severe joint pain and inflammation, and can be triggered by trauma or metabolic stress. - While synovial fluid in gout can have elevated WBCs, it would typically show **negatively birefringent needle-shaped crystals**, which are not mentioned, and the high fever and the presence of a diabetic foot ulcer make septic arthritis a more probable diagnosis. *Direct inoculation of infectious agent* - Direct inoculation usually occurs due to **trauma, surgery, or injection** into the joint. - There is no history of such events in this patient, making hematogenous spread from the existing diabetic foot ulcer a more plausible route for infection.

Question 123: A 75-year-old man presents to his primary care provider with malaise and low-grade fever after he underwent a cystoscopy for recurrent cystitis and pyelonephritis two weeks ago. His past medical history is significant for coronary artery disease and asthma. His current medications include aspirin, metoprolol, atorvastatin, and albuterol inhaler. Temperature is 37.2°C (99.0°F), blood pressure is 110/70 mm Hg, pulse is 92/min and respirations are 14/min. On physical examination, there are painless areas of hemorrhage on his palms and soles. Cardiac auscultation reveals a new pansystolic murmur over the apex. An echocardiogram shows echogenic endocardial vegetation on a leaflet of the mitral valve. Which of the following pathogens is most likely responsible for his condition?

A. Staphylococcus aureus
B. Enterococcus (Correct Answer)
C. Pseudomonas aeruginosa
D. Staphylococcus epidermidis
E. Streptococcus gallolyticus

Explanation: ***Enterococcus*** - The patient's history of recent **cystoscopy**, recurrent **cystitis**, and **pyelonephritis** suggests a genitourinary source of infection, making *Enterococcus* a likely pathogen for **infective endocarditis**. - **Enterococci** are common causes of urinary tract infections, especially in elderly males and those undergoing urological procedures, and can subsequently cause endocarditis. *Staphylococcus aureus* - While *S. aureus* is a leading cause of infective endocarditis, particularly in intravenous drug users or those with central lines, there is no direct evidence here to suggest a **cutaneous** or **catheter-related** entry point. - The patient's clinical presentation, specifically the preceding genitourinary procedure, points away from *S. aureus* as the most probable cause. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* typically causes endocarditis in specific contexts such as **intravenous drug use** or in patients with **prosthetic valves** and healthcare-associated infections, none of which are strongly indicated. - Despite the history of recent instrumentation, *Pseudomonas* is less common as a cause of endocarditis following cystoscopy compared to *Enterococcus*. *Staphylococcus epidermidis* - *S. epidermidis* is usually associated with **prosthetic valve endocarditis** or infection of **intravascular devices**, as it is a common skin commensal. - The patient has no history of prosthetic valves, and the infection appears to be linked to a genitourinary procedure rather than a device-related contamination. *Streptococcus gallolyticus* - *Streptococcus gallolyticus* (**formerly *S. bovis***) is classically associated with **colorectal carcinoma** and gastrointestinal sources of bacteremia. - Although it can cause endocarditis, the patient's presentation with a recent cystoscopy points towards a genitourinary pathogen rather than a gastrointestinal one.

Question 124: A 46-day-old baby is admitted to the pediatric ward with an elevated temperature, erosive periumbilical lesion, clear discharge from the umbilicus, and failure to thrive. She is the first child of a consanguineous couple born vaginally at 38 weeks gestation in an uncomplicated pregnancy. She was discharged home from the nursery within the first week of life without signs of infection or jaundice. The umbilical cord separated at 1 month of age with an increase in temperature and periumbilical inflammation that her mother treated with an herbal decoction. The vital signs are blood pressure 70/45 mm Hg, heart rate 129/min, respiratory rate 26/min, and temperature, 38.9°C (102.0°F). The baby's weight is between the 10th and 5th percentiles and her length is between the 50th and 75th percentiles for her age. The physical examination shows an erosive lesion with perifocal erythema in the periumbilical region with drainage but no pus. The rest of the examination is within normal limits for the patient’s age. The complete blood count shows the following results: Erythrocytes 3.4 x 106/mm3 Hb 11 g/dL Total leukocyte count Neutrophils Lymphocyte Eosinophils Monocytes Basophils 49.200/mm3 61% 33% 2% 2% 2% Platelet count 229,000/mm3 The umbilical discharge culture shows the growth of Staphylococcus aureus. Flow cytometry is performed for suspected primary immunodeficiency. The patient is shown to be CD18-deficient. Which of the following statements best describes the patient’s condition?

A. There is excessive secretion of IL-2 in this patient.
B. The patient’s leukocytes cannot interact with selectins expressed on the surface of endothelial cells.
C. The patient’s leukocytes fail to adhere to the endothelium during their migration to the site of infection. (Correct Answer)
D. The patient’s neutrophils fail to produce reactive oxygen species to destroy engulfed bacteria.
E. The patient has impaired formation of the membrane attack complex.

Explanation: ***The patient’s leukocytes fail to adhere to the endothelium during their migration to the site of infection.*** - The patient has **Leukocyte Adhesion Deficiency Type 1 (LAD1)**, caused by a defect in **CD18**, a subunit of **integrins**. These integrins are crucial for the firm adhesion of leukocytes to the endothelium. - This inability to firmly adhere prevents leukocytes from effectively migrating out of blood vessels into infected tissues, leading to recurrent bacterial infections and impaired wound healing. *There is excessive secretion of IL-2 in this patient.* - **Interleukin-2 (IL-2)** is a cytokine primarily involved in T-cell proliferation and differentiation; its excessive secretion is not characteristic of LAD1. - LAD1 is a defect in cell adhesion, not a dysregulation of cytokine production. *The patient has impaired formation of the membrane attack complex.* - Impaired formation of the **membrane attack complex (MAC)** is characteristic of deficiencies in terminal complement components (C5-C9), leading to increased susceptibility to *Neisseria* infections. - LAD1 involves a defect in leukocyte adhesion, not complement system function. *The patient’s leukocytes cannot interact with selectins expressed on the surface of endothelial cells.* - The initial rolling of leukocytes along the endothelium is mediated by **selectins** (on endothelial cells) interacting with their ligands on leukocytes. - In LAD1, the defect is in **integrins (CD18)**, which are responsible for the **firm adhesion** phase after rolling, not the initial selectin-mediated interaction. *The patient’s neutrophils fail to produce reactive oxygen species to destroy engulfed bacteria.* - Failure to produce **reactive oxygen species (ROS)** is the hallmark of **Chronic Granulomatous Disease (CGD)**, a defect in **NADPH oxidase**, leading to recurrent infections with catalase-positive organisms. - LAD1 is a defect in leukocyte adhesion and migration, not in intracellular killing mechanisms.

Question 125: A 24-year-old man presents to the emergency department for bloody stools. The patient states that he has had bloody diarrhea for the past 3 days without improvement. He recently returned from a camping trip where he drank stream water and admits to eating undercooked meats which included beef, chicken, pork, and salmon. The patient's father died at age 40 due to colon cancer, and his mother died of breast cancer at the age of 52. The patient lives alone and drinks socially. The patient has unprotected sex with multiple male partners. His temperature is 98.3°F (36.8°C), blood pressure is 107/58 mmHg, pulse is 127/min, respirations are 12/min, and oxygen saturation is 99% on room air. Laboratory values are ordered as seen below. Hemoglobin: 9.2 g/dL Hematocrit: 29% Leukocyte count: 9,500/mm^3 with normal differential Platelet count: 87,000/mm^3 Lactate dehydrogenase: 327 IU/L Haptoglobin: 5 mg/dL Serum: Na+: 139 mEq/L Cl-: 100 mEq/L K+: 5.9 mEq/L HCO3-: 19 mEq/L BUN: 39 mg/dL Glucose: 99 mg/dL Creatinine: 1.1 mg/dL Ca2+: 10.2 mg/dL Which of the following is the most likely cause of this patient's presentation?

A. Entamoeba histolytica
B. Campylobacter jejuni
C. Giardia lamblia
D. Colon cancer
E. Escherichia coli (Correct Answer)

Explanation: ***Escherichia coli*** - The patient presents with **bloody diarrhea**, acute **kidney injury (elevated BUN)**, **thrombocytopenia**, and **microangiopathic hemolytic anemia** (indicated by low hemoglobin, low haptoglobin, and elevated LDH), which are classic signs of **hemolytic-uremic syndrome (HUS)**. - HUS is most commonly caused by **Shiga toxin-producing *E. coli* (STEC)**, typically O157:H7, often acquired through consumption of **undercooked meat**. *Entamoeba histolytica* - This parasite causes **amoebic dysentery** (bloody diarrhea), but it typically does not lead to **hemolytic-uremic syndrome (HUS)**. - While it can be acquired through contaminated water, the constellation of hematologic and renal findings (thrombocytopenia, AKI, hemolysis) points away from *E. histolytica*. *Campylobacter jejuni* - *Campylobacter jejuni* is a common cause of **bloody diarrhea** and can be acquired from undercooked poultry. - However, it is more commonly associated with **Guillain-Barré syndrome** and **reactive arthritis**, rather than HUS. *Giardia lamblia* - *Giardia* typically causes **non-bloody, watery diarrhea**, often with **steatorrhea**, and is associated with camping and drinking untreated stream water. - It does not cause bloody diarrhea, thrombocytopenia, hemolytic anemia, or acute kidney injury. *Colon cancer* - While colon cancer can cause **bloody stools** and the patient has a family history, his acute presentation with **severe, acute bloody diarrhea**, **thrombocytopenia**, and **renal injury** is not typical for the initial presentation of colon cancer. - The findings are more indicative of an **acute infectious process** leading to HUS.

Question 126: A 39-year-old man comes to the emergency department because of a 2-day history of fever, chills, dyspnea, and a non-bloody productive cough. He was diagnosed with HIV infection 4 years ago and has been on highly active antiretroviral therapy since then. His temperature is 38.8°C (101.8°F). Examination shows crackles over the left lower lung base. His CD4+ T-lymphocyte count is 520/mm3 (N ≥ 500). An x-ray of the chest shows an infiltrate in the left lower lobe. Sputum cultures grow colonies with a narrow zone of green hemolysis without clearing on blood agar. The most likely causal pathogen of this patient's condition produces which of the following virulence factors?

A. Protein A
B. M protein
C. Polysaccharide capsule (Correct Answer)
D. Type III secretion system
E. Lipopolysaccharide

Explanation: ***Polysaccharide capsule*** - The patient's symptoms (fever, chills, dyspnea, productive cough, crackles, left lower lobe infiltrate) are consistent with **bacterial pneumonia**. The sputum culture showing colonies with a **narrow zone of green hemolysis (alpha-hemolysis)** without clearing on blood agar is characteristic of *Streptococcus pneumoniae*. - *Streptococcus pneumoniae* is encased in a **polysaccharide capsule**, which is its primary virulence factor. This capsule helps the bacteria evade phagocytosis by preventing the attachment of antibodies and complement proteins, allowing it to survive and proliferate in the host. *Protein A* - **Protein A** is a major virulence factor associated with *Staphylococcus aureus*, not *Streptococcus pneumoniae*. - It binds to the Fc region of antibodies, particularly IgG, interfering with opsonization and phagocytosis. *M protein* - **M protein** is a key virulence factor found in *Streptococcus pyogenes* (Group A Streptococcus), which typically causes pharyngitis, scarlet fever, and rheumatic fever. - While *Streptococcus pyogenes* also causes hemolysis, it exhibits **beta-hemolysis** (complete clearing) on blood agar, unlike the alpha-hemolysis seen in this patient's culture. *Type III secretion system* - **Type III secretion systems** are complex protein machines that inject bacterial effector proteins directly into host cells, commonly found in gram-negative bacteria such as *Salmonella*, *Shigella*, and *Pseudomonas*. - *Streptococcus pneumoniae* is a **gram-positive bacterium** and does not possess a Type III secretion system. *Lipopolysaccharide* - **Lipopolysaccharide (LPS)**, also known as **endotoxin**, is a major component of the outer membrane of **gram-negative bacteria**. - *Streptococcus pneumoniae* is a **gram-positive bacterium** and therefore lacks LPS.

Question 127: A 49-year-old male complains of abdominal discomfort that worsens following meals. A gastric biopsy reveals a 2 cm gastric ulcer, and immunohistochemical staining demonstrates the presence of a rod-shaped bacterium in the gastric mucosa. Which of the following is used by the infiltrating pathogen to neutralize gastric acidity?

A. Mucinase
B. LT toxin
C. Flagella
D. Bismuth
E. Urease (Correct Answer)

Explanation: ***Urease*** - The presence of a rod-shaped bacterium in the gastric mucosa causing ulcers points to **Helicobacter pylori**. - **H. pylori** produces **urease**, an enzyme that converts urea into ammonia and carbon dioxide, creating a more alkaline environment around the bacterium, thus neutralizing gastric acid and allowing its survival. *Mucinase* - **Mucinase** is an enzyme produced by some bacteria that breaks down the protective mucus layer, contributing to mucosal damage. - While contributing to pathogenesis, mucinase does not directly neutralize gastric acidity. *LT toxin* - **LT toxin** (heat-labile toxin) is primarily associated with **enterotoxigenic Escherichia coli (ETEC)** and causes watery diarrhea by increasing cAMP. - This toxin is not produced by **H. pylori** and does not play a role in neutralizing gastric acid. *Flagella* - **Flagella** are **locomotor appendages** that allow **H. pylori** to move through the gastric mucus layer and colonize the gastric epithelium. - While crucial for host colonization, flagella do not directly neutralize gastric acid. *Bismuth* - **Bismuth** is a component of some multi-drug regimens used to treat **H. pylori** infection, acting as an antimicrobial and mucosal protectant. - It is not produced by the bacterium but is a medication given to the patient.

Question 128: A 35-year-old female presents to the emergency room complaining of diarrhea and dehydration. She has been experiencing severe watery diarrhea for the past 3 days. She reports that she has been unable to leave the bathroom for more than a few minutes at a time. The diarrhea is profuse and watery without visible blood or mucus. She recently returned from a volunteer trip to Yemen where she worked at an orphanage. Her past medical history is notable for psoriasis for which she takes sulfasalazine. The patient drinks socially and does not smoke. Her temperature is 99°F (37.2°C), blood pressure is 100/55 mmHg, pulse is 130/min, and respirations are 20/min. Mucous membranes are dry. Her eyes appear sunken. Capillary refill is 4 seconds. The patient is started on intravenous fluid resuscitation. Which of the following processes is capable of transmitting the genetic material for the toxin responsible for this patient's condition?

A. Transposition
B. Conjugation
C. Endospore formation
D. Transduction (Correct Answer)
E. Transformation

Explanation: ***Transduction*** - The patient's symptoms are highly suggestive of **cholera**, caused by *Vibrio cholerae*, which produces **cholera toxin**. - The genes for cholera toxin are carried on a **bacteriophage (CTXφ)**, and their transfer between bacteria occurs via **transduction**. *Transposition* - **Transposition** involves the movement of **transposons ("jumping genes")** within a genome or between DNA molecules. - While transposons can carry antimicrobial resistance genes or virulence factors, this mechanism is not typically associated with the transfer of the primary cholera toxin genes. *Conjugation* - **Conjugation** is the transfer of genetic material between bacteria through direct cell-to-cell contact, often involving a **pilus** and the transfer of **plasmids**. - While *Vibrio cholerae* can engage in conjugation, the cholera toxin genes are predominantly acquired via specialized transduction with the CTXφ phage, not typically plasmid-mediated conjugation. *Endospore formation* - **Endospore formation** is a survival mechanism used by certain bacteria (e.g., *Clostridium*, *Bacillus*) to withstand harsh environmental conditions. - It is not a mechanism for **horizontal gene transfer** or the transmission of toxin-encoding genetic material between bacteria. *Transformation* - **Transformation** is the uptake of **naked DNA** from the environment by a bacterial cell. - While *Vibrio cholerae* can be naturally competent for transformation, the cholera toxin genes are primarily acquired through **phage-mediated transduction**, not free DNA uptake.

Question 129: A 72-year-old woman comes to the physician because of a 3-day history of redness and swelling of her right leg and fever. She says the leg is very painful and the redness over it has become larger. She appears ill. Her temperature is 39.3°C (102.7°F), pulse is 103/min, and blood pressure is 138/90 mm Hg. Cardiopulmonary examination shows no abnormalities. Examination shows an area of diffuse erythema and swelling over her anterior right lower leg; it is warm and tender to touch. Squeezing of the calf does not elicit tenderness. There is swelling of the right inguinal lymph nodes. Pedal pulses are palpable bilaterally. Which of the following is the strongest predisposing factor for this patient's condition?

A. Cigarette smoking
B. Graves disease
C. Tinea pedis (Correct Answer)
D. Rheumatoid arthritis
E. Immobility

Explanation: ***Tinea pedis*** - The patient's presentation of **unilateral leg redness, swelling, warmth, tenderness, and fever** is highly suggestive of **bacterial cellulitis**. - **Tinea pedis (athlete's foot)** causes breaks in the skin barrier, allowing bacteria (typically *Streptococcus pyogenes* or *Staphylococcus aureus*) to enter and cause infection. *Cigarette smoking* - While smoking has numerous negative health effects, it is **not a direct predisposing factor for cellulitis**. - Smoking can impair wound healing and immune function generally, but it does not specifically increase the risk of skin barrier breakdown in the way fungal infections do. *Graves disease* - **Graves disease** is an autoimmune condition causing hyperthyroidism and is not directly linked to an increased risk of cellulitis. - It can cause **pretibial myxedema**, which involves skin changes but does not typically lead to skin breakdown and bacterial infection. *Rheumatoid arthritis* - **Rheumatoid arthritis** is a chronic inflammatory autoimmune disease primarily affecting joints. - While patients on immunosuppressive therapy for RA may have a higher risk of infections in general, RA itself does not directly predispose to cellulitis by causing skin barrier disruption. *Immobility* - **Immobility** can lead to conditions like **deep vein thrombosis (DVT)** or **pressure ulcers**, but it is not the strongest direct predisposing factor for cellulitis in this clinical scenario. - While immobility often accompanies conditions that impair skin integrity (e.g., venous insufficiency), it doesn't directly cause the primary break in the skin that leads to cellulitis as Tinea pedis does.

Question 130: A 20-year-old woman presents to student health for a 7-day history of sinus congestion. She has also had fever, sore throat, and infectious gastroenteritis. Upon further questioning, she has had similar problems 2 or 3 times a year for as long as she can remember. These have included sinus infections, ear infections, and lung infections. At the clinic, her temperature is 38.6°C (101.4°F), heart rate is 70/min, blood pressure is 126/78 mm Hg, respiratory rate is 18/min, and oxygen saturation is 98% on room air. Physical examination is notable for mucopurulent discharge from both nares and tenderness to palpation over her bilateral maxillae. Sputum gram stain shows gram-positive diplococci. Which of the following best describes the levels of immunoglobulins that would most likely be found upon testing this patient's serum?

A. IgM Level: Low, IgG Level: Low, IgA Level: Low
B. IgM Level: Normal, IgG Level: Low, IgA Level: Low
C. IgM Level: Elevated, IgG Level: Low, IgA Level: Low
D. IgM Level: Normal, IgG Level: Normal, IgA Level: Low (Correct Answer)
E. IgM Level: Normal, IgG Level: Normal, IgA Level: Normal

Explanation: ***IgM Level: Normal, IgG Level: Normal, IgA Level: Low*** - The recurrent sinopulmonary and gastrointestinal infections, along with the chronic nature starting from childhood, are highly suggestive of **Selective IgA Deficiency (SIgAD)**. Individuals with SIgAD typically have normal levels of IgM and IgG. - **Selective IgA deficiency** is the most common primary immunodeficiency; its key characteristic is isolated low or absent IgA with otherwise normal levels of other immunoglobulins. *IgM Level: Low, IgG Level: Low, IgA Level: Low* - This profile, involving deficiencies in all major immunoglobulin classes, would indicate a more severe combined immunodeficiency, such as **Severe Combined Immunodeficiency (SCID)** or **Common Variable Immunodeficiency (CVID)**. - While patients with these conditions also experience recurrent infections, the typical presentation is often more severe and may include failure to thrive, which is not suggested in this case of a 20-year-old woman. *IgM Level: Normal, IgG Level: Low, IgA Level: Low* - While IgA is low, a low IgG in addition to a low IgA would point towards conditions like **Common Variable Immunodeficiency (CVID)**, which affects multiple immunoglobulin classes. - CVID typically presents with more polymorphic clinical features and a broader range of infections, often later in life, although recurrent sinopulmonary infections are common. *IgM Level: Elevated, IgG Level: Low, IgA Level: Low* - An elevated IgM with low IgG and IgA is characteristic of **Hyper-IgM Syndrome**. - This syndrome is usually associated with recurrent infections, often opportunistic, and can also lead to neutropenia, which is not indicated in the provided clinical picture. *IgM Level: Normal, IgG Level: Normal, IgA Level: Normal* - This immunoglobulin profile represents a **healthy immune system**. - Given the patient's history of recurrent and chronic infections, it is highly unlikely that her immunoglobulin levels are all normal.

Question 131: A 6-month-old girl presents with recurring skin infections. Past medical history is significant for 3 episodes of acute otitis media since birth. The patient was born at 39 weeks via an uncomplicated, spontaneous transvaginal delivery, but there was delayed umbilical cord separation. She has met all developmental milestones. On physical examination, the skin around her mouth is inflamed and red. Which of the following is most likely responsible for this child’s clinical presentation?

A. Deficiency in NADPH oxidase
B. IL-12 receptor deficiency
C. Absence of CD18 molecule on the surface of leukocytes (Correct Answer)
D. A microtubule dysfunction
E. Defect in tyrosine kinase

Explanation: ***Absence of CD18 molecule on the surface of leukocytes*** - The combination of **recurrent bacterial skin infections**, **otitis media**, and **delayed umbilical cord separation** is highly characteristic of **leukocyte adhesion deficiency type 1 (LAD-1)**. - LAD-1 is caused by a defect in the **CD18 subunit of integrins**, leading to impaired leukocyte extravasation to sites of infection. *Deficiency in NADPH oxidase* - This defect is associated with **chronic granulomatous disease (CGD)**, which presents with recurrent infections by **catalase-positive organisms** and granuloma formation. - While recurrent infections occur, **delayed umbilical cord separation** is not a typical feature of CGD. *IL-12 receptor deficiency* - This deficiency leads to impaired cellular immunity, particularly against **intracellular bacteria (e.g., mycobacteria)** and **fungi**. - It does not typically present with the specific combination of **pyogenic infections** and **delayed umbilical cord separation** seen in this patient. *A microtubule dysfunction* - Microtubule dysfunction can be seen in conditions like **Chédiak-Higashi syndrome**, which involves impaired lysosomal trafficking and leads to **recurrent pyogenic infections**, **oculocutaneous albinism**, and **neuropathy**. - **Delayed umbilical cord separation** is not a characteristic feature of Chédiak-Higashi syndrome. *Defect in tyrosine kinase* - Defects in tyrosine kinase, such as **Bruton's tyrosine kinase (BTK)**, cause **X-linked agammaglobulinemia**, leading to recurrent infections with **encapsulated bacteria** due to impaired B cell development. - This condition is characterized by a lack of mature B cells and **low immunoglobulin levels**, but it does not typically present with **delayed umbilical cord separation**.

Question 132: A 46-year-old obese man comes to the emergency room because of paresthesias in his feet and a hypopigmented skin lesion on his knee that he first noticed 6 weeks ago. He has also had fever, fatigue, and malaise for the last week. He has a history of chronic autoimmune thyroiditis for which he takes levothyroxine. He immigrated from Indonesia 3 years ago to join his family in the United States. His temperature is 38.7°C (101.7°F) and blood pressure is 122/84 mm Hg. Physical exam shows a well-defined hypopigmented skin lesion approximately 3 cm in diameter over the anterior aspect of the right knee. The area has no hair growth and remains dry although he is diaphoretic. There is diminished sensation to light touch and pinprick in the skin lesion when compared to surrounding skin. There is reduced light touch sensation in the big toes bilaterally. After obtaining a skin biopsy of the lesion to confirm the diagnosis, which of the following is the most appropriate initial pharmacotherapy?

A. Topical fluconazole
B. Oral rifampicin and dapsone (Correct Answer)
C. Topical betamethasone
D. Intravenous amphotericin
E. Oral hydroxychloroquine

Explanation: ***Oral rifampicin and dapsone*** - The patient's symptoms (hypopigmented, anesthetic skin lesion, paresthesias, fever, fatigue) and history (immigrant from **Indonesia**, an endemic area for leprosy) are classic for **leprosy (Hansen's disease)**. - For **paucibacillary leprosy**, treatment typically involves **rifampicin** and **dapsone** for 6 months. *Topical fluconazole* - **Fluconazole** is an antifungal medication used to treat fungal infections, such as candidiasis. - The patient's clinical presentation is not consistent with a fungal infection and would not address the neurological symptoms. *Topical betamethasone* - **Betamethasone** is a potent topical corticosteroid used to reduce inflammation and itching in various dermatological conditions. - While it might temporarily alleviate skin inflammation, it would not treat the underlying mycobacterial infection or peripheral neuropathy. *Intravenous amphotericin* - **Amphotericin B** is a broad-spectrum antifungal medication reserved for severe systemic fungal infections. - It is not indicated for bacterial infections like leprosy, and its intravenous administration carries significant side effects. *Oral hydroxychloroquine* - **Hydroxychloroquine** is an antimalarial drug with immunomodulatory properties, used in conditions like lupus and rheumatoid arthritis. - It has no role in the treatment of leprosy.

Question 133: A 41-year-old woman presents to the emergency room with a fever. She has had intermittent fevers accompanied by malaise, weakness, and mild shortness of breath for the past 2 weeks. Her past medical history is notable for recurrent bloody diarrhea for over 3 years. She underwent a flexible sigmoidoscopy several months ago which demonstrated contiguously granular and hyperemic rectal mucosa. She has a distant history of intravenous drug use but has been sober for the past 15 years. Her temperature is 100.8°F (38.2°C), blood pressure is 126/76 mmHg, pulse is 112/min, and respirations are 17/min. On exam, she appears lethargic but is able to answer questions appropriately. A new systolic II/VI murmur is heard on cardiac auscultation. Subungual hemorrhages are noted. Multiple blood cultures are drawn and results are pending. Which of the following pathogens is most strongly associated with this patient's condition?

A. Staphylococcus epidermidis
B. Streptococcus viridans
C. Streptococcus gallolyticus (Correct Answer)
D. Candida albicans
E. Pseudomonas aeruginosa

Explanation: ***Streptococcus gallolyticus*** - The patient's history of **intermittent fevers**, a **new cardiac murmur**, and **subungual hemorrhages** are classic signs of **infective endocarditis**. - **_Streptococcus gallolyticus_** (formerly _Streptococcus bovis_) is strongly associated with infective endocarditis in patients with concomitant **gastrointestinal pathology**, particularly **colorectal cancer** or other inflammatory bowel conditions, which aligns with her history of chronic bloody diarrhea and contiguous granular and hyperemic rectal mucosa suggestive of **ulcerative colitis**. *Staphylococcus epidermidis* - This pathogen is a common cause of **prosthetic valve endocarditis** or catheter-related infections due to its ability to form biofilms on foreign materials. - The patient in this case has no history of prosthetic valves or recent catheter use, making this less likely. *Streptococcus viridans* - **_Streptococcus viridans_** is a common cause of **native valve endocarditis**, especially after dental procedures, and is often associated with poor oral hygiene. - While it can cause endocarditis, the strong association with **gastrointestinal disease** in this patient points more specifically to _Streptococcus gallolyticus_. *Candida albicans* - **_Candida albicans_** can cause endocarditis, particularly in immunocompromised individuals, those with indwelling catheters, or intravenous drug users. - Although the patient has a distant history of IV drug use, the current presentation does not strongly suggest fungemia, and the GI link is a more prominent feature. *Pseudomonas aeruginosa* - **_Pseudomonas aeruginosa_** endocarditis is typically seen in patients with **nosocomial infections**, **intravenous drug use**, or compromised immune systems. - While her distant IV drug use history is noted, the primary clues (GI pathology, classic endocarditis signs) do not specifically point to _Pseudomonas_.

Question 134: A 33-year-old man presents to the emergency department with a fever and fatigue. He states that he has not felt well since he returned from a hiking trip in Alabama. He is generally healthy and has no other medical conditions. His temperature is 101°F (38.3°C), blood pressure is 127/85 mmHg, pulse is 108/min, respirations are 14/min, and oxygen saturation is 99% on room air. Physical exam including a full dermatologic inspection is unremarkable. Laboratory studies are ordered as seen below. Hemoglobin: 13 g/dL Hematocrit: 39% Leukocyte count: 2,200/mm^3 with normal differential Platelet count: 77,000/mm^3 Serum: Na+: 139 mEq/L Cl-: 100 mEq/L K+: 4.3 mEq/L HCO3-: 24 mEq/L BUN: 19 mg/dL Glucose: 98 mg/dL Creatinine: 1.3 mg/dL Ca2+: 10.2 mg/dL AST: 92 U/L ALT: 100 U/L Which of the following is the most likely diagnosis?

A. Lyme disease
B. Babesiosis
C. Influenza
D. Ehrlichiosis (Correct Answer)
E. Rocky Mountain spotted fever

Explanation: ***Ehrlichiosis*** - This patient's symptoms (fever, fatigue), recent travel to an **endemic area** (Alabama), **leukopenia** (WBC 2,200/mm^3), **thrombocytopenia** (platelet 77,000/mm^3), and **elevated liver enzymes** (AST 92, ALT 100) are highly characteristic of ehrlichiosis, a **tick-borne disease**. - The absence of a rash helps differentiate it from some other tick-borne illnesses. *Lyme disease* - While Lyme disease is also tick-borne, it typically presents with an **erythema migrans rash**, which is absent in this case. - Lyme disease is less commonly associated with the **pronounced leukopenia** and **thrombocytopenia** seen here. *Rocky Mountain spotted fever* - Rocky Mountain Spotted Fever (RMSF) is characterized by a **maculopapular rash** that often starts on the ankles and wrists and spreads centrally, involving the palms and soles. This rash is absent in the patient. - While RMSF can cause thrombocytopenia and elevated liver enzymes, the **lack of rash is a key differentiator**. *Babesiosis* - Babesiosis is a tick-borne parasitic infection that causes **hemolytic anemia**, which is not clearly indicated by the patient's hemoglobin and hematocrit, and typically results in severe fatigue and sometimes splenomegaly. - This condition is often seen in immunocompromised individuals or those without a spleen, and the labs here are more consistent with ehrlichiosis than babesiosis. *Influenza* - Influenza presents with fever, fatigue, myalgia, and respiratory symptoms, but it does not cause **thrombocytopenia**, **leukopenia**, or **elevated liver enzymes** to the extent seen in this patient. - The symptoms are more indicative of a **tick-borne illness** given the travel history and specific lab abnormalities.

Question 135: A 42-year-old man presents to the emergency department with a 3-day history of fever and severe back pain. The fever is high-grade, continuous, without chills and rigors. The back pain is severe, localized to the thoracic region, and aggravated by deep breathing. The patient tried taking ibuprofen with little improvement. Past medical history is significant for essential hypertension, dyslipidemia, hyperuricemia, and bronchial asthma. Current medicines include allopurinol, amlodipine, atorvastatin, clopidogrel, montelukast, and a corticosteroid inhaler. The patient reports a 25-pack-year smoking history and drinks alcohol only socially. His vital signs include: blood pressure 152/94 mm Hg, pulse 101/min, temperature 39.5°C (103.1°F). BMI 36.8 kg/m2. On physical examination, the patient is alert and oriented. Multiple injection marks are visible around the left-sided cubital fossa and hand veins. The neck is supple on head flexion. Point tenderness is present in the thoracic region at the midline. Motor and sensory examinations are unremarkable with normal deep tendon reflexes. Laboratory findings are significant for the following: Hemoglobin 14.5 mg/dL White blood cell 24,500/mm3 Platelets 480,000/mm3 BUN 28 mg/dL Creatinine 1.1 mg/dL ESR 45 mm/hr C-reactive protein 84 mg/dL Sodium 144 mEq/L Potassium 4.1 mEq/L Calcium 9.7 mEq/L A contrast MRI of the spine reveals a peripherally enhancing dorsal epidural process compressing the thecal sac and causing a mild leftwards displacement. Which of the following is the most likely risk factor for this patient’s condition?

A. Alcohol use
B. Increased BMI
C. Inhaled steroid use
D. Intravenous drug use (Correct Answer)
E. Smoking

Explanation: ***Intravenous drug use*** - The patient presents with **fever**, **severe back pain**, and an MRI showing a **peripherally enhancing dorsal epidural process** compressing the thecal sac, which is highly suggestive of a **spinal epidural abscess**. - The presence of **multiple injection marks** around the cubital fossa and hand veins, along with elevated inflammatory markers (**WBC 24,500/mm3**, **ESR 45 mm/hr**, **CRP 84 mg/dL**), strongly indicates **intravenous drug use (IVDU)** as the most likely risk factor, as it introduces bacteria directly into the bloodstream, leading to hematogenous spread to the spine. *Alcohol use* - While alcohol use can weaken the immune system and contribute to various infections, it is **not a direct risk factor** for **spinal epidural abscesses** in the same way intravenous drug use is, which directly introduces pathogens into the bloodstream. - The patient reports only **social alcohol use**, making it a less likely primary contributor to such a severe infection compared to the clear signs of IVDU. *Increased BMI* - **Obesity (BMI 36.8 kg/m2)** is a risk factor for many health issues, including chronic back pain and surgical complications, but it is **not a specific or direct risk factor** for developing a **spinal epidural abscess**. - While obesity can be associated with a compromised immune system, there is no direct causal link between increased BMI and the hematogenous spread of infection leading to an epidural abscess. *Inhaled steroid use* - **Inhaled corticosteroids** used for asthma are associated with localized effects and have **minimal systemic immunosuppressive effects** compared to oral or injected steroids. - Therefore, their use is **unlikely to significantly predispose** the patient to a severe systemic infection like a spinal epidural abscess. *Smoking* - **Smoking** is a well-known risk factor for various health problems, including respiratory infections and general immune compromise. - However, similar to alcohol use and increased BMI, it is **not as direct or significant a risk factor** for a **spinal epidural abscess** as the direct introduction of bacteria into the bloodstream via intravenous drug use.

Question 136: A 24-year-old man presents with a painless genital ulcer for the past 2 weeks. He reports that he recently has been having unprotected sex with multiple partners. Past medical history is unremarkable. On physical examination, a single ulcer is present on the dorsal shaft of the penis which is circumscribed, indurated, and partially healed. There is moderate inguinal lymphadenopathy but no buboes. Which of the following tests would confirm the most likely diagnosis in this patient?

A. Perform a darkfield microscopic examination of a swab from the chancre (Correct Answer)
B. Swab the chancre and perform a saline wet mount
C. Fluorescent treponemal antibody absorption (FTA-ABS) test
D. Frei test
E. Venereal Disease Research Laboratory (VDRL) test

Explanation: ***Perform a darkfield microscopic examination of a swab from the chancre*** - This patient's presentation with a **painless, indurated genital ulcer** (chancre) and regional lymphadenopathy is highly suggestive of **primary syphilis**. - **Darkfield microscopy** directly visualizes the spirochetes (**_Treponema pallidum_**) from the chancre, providing a definitive and rapid diagnosis. *Swab the chancre and perform a saline wet mount* - A **saline wet mount** is used to identify mobile organisms like **_Trichomonas vaginalis_** or clue cells suggestive of **bacterial vaginosis**, neither of which are associated with this type of ulcer. - This test would not reveal the spirochetes responsible for syphilis and is not appropriate for diagnosing a genital ulcer. *Fluorescent treponemal antibody absorption (FTA-ABS) test* - The **FTA-ABS test** is a **treponemal-specific serological test** used to confirm a syphilis diagnosis, particularly in later stages or when non-treponemal tests are reactive. - While sensitive for syphilis, it's typically reactive later in the disease course and cannot directly visualize the bacteria from the ulcer, making darkfield microscopy a more immediate and direct diagnostic tool for primary syphilis. *Frei test* - The **Frei test** is an obsolete intradermal skin test used to diagnose **lymphogranuloma venereum (LGV)**, which typically presents with a transient, unnoticed lesion followed by severe lymphadenopathy and buboes. - It is not used for the diagnosis of syphilis and would not be helpful for this patient's presentation. *Viral and rickettsial disease research laboratory (VDRL) test* - The **VDRL test** is a **non-treponemal serological test** for syphilis that detects antibodies against cardiolipin, a lipid released from damaged host cells. - While used for screening and monitoring treatment response, it can be **negative in early primary syphilis** (before seroconversion) and may not be positive at the time of presentation with a fresh chancre.

Question 137: A 1-year-old girl is brought to the physician for a well-child examination. She has no history of serious illness. She receives a vaccine in which a polysaccharide is conjugated to a carrier protein. Which of the following pathogens is the most likely target of this vaccine?

A. Hepatitis A virus
B. Varicella zoster virus
C. Streptococcus pneumoniae (Correct Answer)
D. Bordetella pertussis
E. Clostridium tetani

Explanation: ***Streptococcus pneumoniae*** - This pathogen is a common cause of **pneumonia**, **otitis media**, and **meningitis** in young children. The **pneumococcal conjugate vaccine (PCV)** targets *Streptococcus pneumoniae*'s polysaccharide capsule by conjugating it to a carrier protein. - Conjugating the polysaccharide to a protein carrier allows for a **T-cell-dependent immune response**, which is crucial for eliciting a robust and long-lasting antibody response in infants and young children, whose immune systems are not yet mature enough to respond effectively to unconjugated polysaccharide antigens. *Hepatitis A virus* - The vaccine for **Hepatitis A virus** is an **inactivated vaccine** containing whole killed virus particles, not a polysaccharide conjugated to a carrier protein. - It is typically given to children to prevent **Hepatitis A infection**, which causes liver inflammation. *Varicella zoster virus* - The **varicella vaccine** for **Varicella zoster virus** is a **live, attenuated vaccine**, meaning it contains a weakened form of the live virus. - This vaccine aims to prevent **chickenpox** and is not a polysaccharide-protein conjugate vaccine. *Bordetella pertussis* - The vaccine for **Bordetella pertussis** (whooping cough) is part of the **DTaP vaccine** and is an **acellular vaccine**, containing purified components of the bacterium. - These components are primarily **toxoids** (inactivated toxins) or other bacterial proteins, not polysaccharides. *Clostridium tetani* - The vaccine for **Clostridium tetani** is a **toxoid vaccine**, meaning it contains an inactivated form of the **tetanus toxin**. - This is part of the **DTaP vaccine** and works by stimulating an immune response against the toxin, not bacterial polysaccharides.

Question 138: A 10-year-old girl presents to your office with a fever and rash. Her mother first noticed the rash 2 days ago after a camping trip. The rash began on her wrists and ankles and has now spread to her palms and the soles of her feet. This morning, she was feeling unwell and complaining of a headache. She had a fever of 102°F (39°C) prompting her mother to bring her to your office. She is otherwise healthy and does not take any medications. Her medical history is significant for a broken arm at age 8. On physical exam her blood pressure is 120/80 mmHg, pulse is 110/min, and respirations are 22/min. You notice a petechial rash on the palms, soles, ankles, and wrists. Which of the following findings would confirm the most likely cause of this patient's symptoms?

A. Granulocytes with morulae in the cytoplasm
B. Cross-reactivity of serum with proteus antigens (Correct Answer)
C. Monocytes with morulae in the cytoplasm
D. Positive Borrelia burgdorferi antibodies
E. Positive fluorescent treponemal antibody absorption test (FTA-ABS)

Explanation: ***Cross-reactivity of serum with proteus antigens*** - The patient's presentation (fever, rash starting on wrists/ankles and spreading to palms/soles, petechiae, headache, camping/tick exposure history) is **classic for Rocky Mountain Spotted Fever (RMSF)**, caused by *Rickettsia rickettsii*. - The **Weil-Felix test** detects antibodies that cross-react with *Proteus vulgaris* antigens (OX-19 and OX-2), and has been historically used to diagnose **rickettsial infections** including RMSF. - While modern diagnosis relies on **immunofluorescence assays (IFA)** or **PCR**, the Weil-Felix test represents the classic serological finding. Due to low sensitivity and specificity, it has largely been replaced but remains a recognized diagnostic marker for RMSF. - **Important**: RMSF requires **prompt empiric treatment with doxycycline** without waiting for confirmatory testing, as delay can be fatal. *Granulocytes with morulae in the cytoplasm* - The presence of **morulae** (mulberry-like clusters) within **granulocytes** is characteristic of **anaplasmosis**, caused by *Anaplasma phagocytophilum*. - While anaplasmosis is also a **tick-borne illness** with fever and headache, the **rash is uncommon** and when present does **not have the characteristic centripetal distribution** (wrists/ankles → palms/soles) seen in RMSF. - Morulae are **NOT seen in RMSF**, which involves endothelial cell infection. *Positive Borrelia burgdorferi antibodies* - *Borrelia burgdorferi* causes **Lyme disease**, which typically presents with **erythema migrans** (bull's-eye rash), fever, and later arthralgia or cardiac/neurologic complications. - The rash in this case (petechial, starting on extremities and spreading to palms/soles) is **not consistent with erythema migrans**, which is an expanding erythematous patch with central clearing. *Monocytes with morulae in the cytoplasm* - The presence of morulae within the **cytoplasm of monocytes** is characteristic of **ehrlichiosis**, caused by *Ehrlichia chaffeensis*. - Ehrlichiosis is also a **tick-borne illness** with similar symptoms (fever, headache), but **rash is less common** (about 30% of cases) and does not typically show the centripetal distribution characteristic of RMSF. - Like anaplasmosis, morulae are **not seen in RMSF**. *Positive fluorescent treponemal antibody absorption test (FTA-ABS)* - The **FTA-ABS test** is a specific confirmatory test for **syphilis**, caused by *Treponema pallidum*. - Syphilis has a completely different presentation (painless chancre in primary, maculopapular rash including palms/soles in secondary) and is **sexually transmitted**, not tick-borne.

Question 139: A 45-year-old woman comes to the emergency department because of abdominal cramping, vomiting, and watery diarrhea for the past 4 hours. One day ago, she went to a seafood restaurant with her family to celebrate her birthday. Three of the attendees have developed similar symptoms. The patient appears lethargic. Her temperature is 38.8°C (101.8°F). Which of the following organisms is most likely responsible for this patient's current symptoms?

A. Vibrio parahaemolyticus (Correct Answer)
B. Staphylococcus aureus
C. Salmonella enterica
D. Campylobacter jejuni
E. Listeria monocytogenes

Explanation: ***Vibrio parahaemolyticus*** - This organism is commonly associated with the consumption of **raw or undercooked seafood** and causes **acute gastroenteritis** with vomiting, watery diarrhea, and abdominal cramps. - The **24-hour incubation period** (symptoms began 4 hours ago after eating seafood 1 day ago) fits well with *V. parahaemolyticus*, which typically has an incubation of **12-24 hours** (range 4-96 hours). - The involvement of multiple individuals who ate at the same seafood restaurant strongly points to a **foodborne infection** from contaminated seafood. - The presence of fever (38.8°C) is consistent with *V. parahaemolyticus* gastroenteritis. *Staphylococcus aureus* - *S. aureus* causes food poisoning with a very **short incubation period (1-6 hours)** due to preformed enterotoxin, which does **not** match the 24-hour timeline in this case. - While it can cause rapid-onset vomiting and diarrhea, it is more commonly associated with contaminated **dairy products, mayonnaise salads, or meats**, not typically seafood. - Fever is uncommon in *S. aureus* enterotoxin-mediated food poisoning. *Salmonella enterica* - *Salmonella* infections typically have an incubation period of **6-72 hours** (often 12-36 hours) and could fit the timeline, but are more commonly associated with **poultry, eggs, or contaminated produce** rather than seafood as the primary source. - While it causes fever, vomiting, and diarrhea, the **seafood exposure** makes *Vibrio parahaemolyticus* the more likely pathogen. *Campylobacter jejuni* - *Campylobacter jejuni* usually causes **inflammatory diarrhea** (often bloody) with an incubation period of **2-5 days**, which is longer than the 24-hour period in this case. - It is commonly linked to **undercooked poultry or unpasteurized milk**, not typically seafood. - The watery (non-bloody) diarrhea presentation also makes this less likely. *Listeria monocytogenes* - *Listeria monocytogenes* is associated with **deli meats, soft cheeses, and unpasteurized dairy products**, and has a much longer and highly variable incubation period **(1-70 days, median ~3 weeks)**. - While it can cause gastroenteritis, its primary concern is severe invasive disease in immunocompromised individuals, pregnant women, and the elderly. - The rapid 24-hour onset with seafood exposure does not fit *Listeria*.

Question 140: A child is brought into the emergency room by her mother. Her mother states that the 7-year-old child was playing with their dog, who is up to date on his vaccinations. When the dog started playing more aggressively, the child suffered a bite on the hand with two puncture wounds from the dog's canines. The child is up-to-date on her vaccinations and has no medical history. Her vitals are within normal limits. If this bite becomes infected, what is the most likely organism to be the cause of infection?

A. Clostridium tetani
B. Fusobacterium
C. Pasteurella multocida (Correct Answer)
D. Clostridium perfringens
E. Pseudomonas aeruginosa

Explanation: ***Pasteurella multocida*** - This bacterium is a **common commensal** in the oral cavity of cats and dogs and is the **most frequent cause** of wound infections following animal bites. - Infections with *Pasteurella multocida* typically present rapidly within **24 hours** with **erythema, swelling, tenderness, and purulent discharge** at the bite site. *Clostridium tetani* - This organism causes **tetanus**, characterized by **muscle spasms and lockjaw**, not focal wound infection, and is typically associated with penetrating injuries contaminated with soil. - The child is stated to be **up-to-date on vaccinations**, making tetanus highly unlikely. *Fusobacterium* - *Fusobacterium* species are anaerobic bacteria often found in the oral cavity and can cause polymicrobial infections, but they are **not the primary or most likely organism** in dog bite infections. - They are more commonly associated with **periodontal disease** and certain severe, invasive infections like Lemierre's syndrome. *Clostridium perfringens* - This bacterium is a significant cause of **gas gangrene** (clostridial myonecrosis), which is characterized by rapid tissue necrosis and gas production. - While it can be associated with contaminated wounds, it is **not the most common pathogen** in dog bites and presents with a more severe, systemic picture. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is an opportunistic pathogen often associated with **water contamination**, **hospital-acquired infections**, or infections in immunocompromised individuals. - It is **not a primary pathogen** in the oral flora of dogs and therefore uncommonly causes dog bite infections.

Question 141: A 25-year-old G1P0 female of Middle Eastern descent gives birth to a male infant at 38 weeks' gestation. The child is in the 15th percentile for both height and weight. Over the course of the first six months of the child's life, he develops multiple severe skin and mucosal infections characterized by dramatically elevated white blood cell counts. The patient also demonstrates mental retardation soon after birth. A thorough hematologic analysis via flow cytometry reveals that the child's neutrophils lack Sialyl-Lewis X. Which of the following processes is likely deficient in this patient?

A. Chemoattraction
B. Diapedesis
C. Transmigration through the extracellular matrix
D. Rolling (Correct Answer)
E. Tight adhesion

Explanation: ***Rolling*** - The absence of **Sialyl-Lewis X** on neutrophils impairs their ability to bind to **P-selectin** and **E-selectin** on endothelial cells, which is crucial for the initial rolling phase of leukocyte extravasation. - This deficiency is characteristic of **Leukocyte Adhesion Deficiency Type 2 (LAD2)**, also known as **congenital disorder of glycosylation IIc (CDGIIc)**, leading to recurrent infections and developmental delays due to impaired leukocyte trafficking to sites of infection. *Chemoattraction* - **Chemoattraction** involves leukocytes moving along a chemical gradient towards inflammatory signals, which occurs after initial rolling and adhesion. - While essential for reaching the site of infection, the primary defect here is in the initial binding and rolling, rather than the ability to respond to chemokines once bound. *Diapedesis* - **Diapedesis**, or transmigration, is the process where leukocytes squeeze between endothelial cells into the tissue. - This step occurs after stable adhesion, and while it would eventually be inhibited by preceding adhesion defects, it is not the primary process directly affected by the lack of Sialyl-Lewis X. *Transmigration through the extracellular matrix* - **Transmigration through the extracellular matrix (ECM)** is the final step where leukocytes move once they are in the tissue, navigating the ECM to reach the infectious agent. - This process is dependent on successful extravasation and is distinct from the initial adhesion and rolling mechanisms affected by Sialyl-Lewis X deficiency. *Tight adhesion* - **Tight adhesion** involves firm binding between leukocyte integrins and endothelial cell adhesion molecules (e.g., ICAM-1) and occurs after rolling. - Although LAD2 may indirectly affect tight adhesion by preventing sufficient rolling and subsequent integrin activation, the direct defect in Sialyl-Lewis X impacts the initial weak interactions (rolling) rather than the strong, stable adhesive bonds.

Question 142: A scientist is studying the mechanisms by which bacteria become resistant to antibiotics. She begins by obtaining a culture of vancomycin-resistant Enterococcus faecalis and conducts replicate plating experiments. In these experiments, colonies are inoculated onto a membrane and smeared on 2 separate plates, 1 containing vancomycin and the other with no antibiotics. She finds that all of the bacterial colonies are vancomycin resistant because they grow on both plates. She then maintains the bacteria in liquid culture without vancomycin while she performs her other studies. Fifteen generations of bacteria later, she conducts replicate plating experiments again and finds that 20% of the colonies are now sensitive to vancomycin. Which of the following mechanisms is the most likely explanation for why these colonies have become vancomycin sensitive?

A. Point mutation
B. Gain of function mutation
C. Viral infection
D. Plasmid loss (Correct Answer)
E. Loss of function mutation

Explanation: ***Plasmid loss*** - The initial **vancomycin resistance** in *Enterococcus faecalis* is often mediated by genes located on **plasmids**, which are extrachromosomal DNA. - In the absence of selective pressure (vancomycin), bacteria that lose the plasmid (and thus the resistance genes) have a **growth advantage** over those that retain the energetically costly plasmid, leading to an increase in sensitive colonies over generations. *Point mutation* - A **point mutation** typically involves a change in a single nucleotide and could lead to loss of resistance if it occurred in a gene conferring resistance. - However, since there was no selective pressure for loss of resistance, it is less likely that 20% of the population would acquire such a specific point mutation to revert resistance. *Gain of function mutation* - A **gain of function mutation** would imply that the bacteria acquired a *new* advantageous trait, not the *loss* of resistance. - This type of mutation would not explain why some colonies became sensitive to vancomycin after the drug was removed. *Viral infection* - **Viral infection** (bacteriophages) can transfer genes through transduction or cause bacterial lysis, but it's not the primary mechanism for a widespread reversion of resistance in the absence of antibiotic pressure. - It would not explain the observed increase in vancomycin-sensitive colonies due to evolutionary pressure. *Loss of function mutation* - While a **loss of function mutation** in a gene conferring resistance could lead to sensitivity, it's generally less likely to explain a 20% shift without selective pressure than **plasmid loss**. - Plasmids are often unstable and are easily lost in the absence of selection, whereas a specific gene mutation causing loss of function would need to arise and become prevalent in the population.

Question 143: A 48-year-old man comes to the physician because of a hypopigmented skin lesion on his finger. He first noticed it 4 weeks ago after cutting his finger with a knife while preparing food. He did not feel the cut. For the past week, he has also had fever, fatigue, and malaise. He has not traveled outside the country since he immigrated from India to join his family in the United States 2 years ago. His temperature is 38.7°C (101.7°F). Physical examination shows a small, healing laceration on the dorsal aspect of the left index finger and an overlying well-defined, hypopigmented macule with raised borders. Sensation to pinprick and light touch is decreased over this area. Which of the following is the most likely causal pathogen of this patient's condition?

A. Epidermophyton floccosum
B. Mycobacterium leprae (Correct Answer)
C. Malassezia furfur
D. Pseudomonas aeruginosa
E. Leishmania donovani

Explanation: ***Mycobacterium leprae*** - The patient's history of immigration from **India**, a country endemic for leprosy, coupled with the **hypopigmented, anesthetic skin lesion** with raised borders, is classic for **leprosy** (Hansen's disease). - The diminished sensation to pinprick and light touch in the affected area points to **nerve involvement**, a hallmark of *Mycobacterium leprae* infection. *Epidermophyton floccosum* - This fungus primarily causes **tinea infections** (ringworm), such as athlete's foot and jock itch. - While it can cause skin lesions, they are typically **erythematous** and scaly, not hypopigmented with sensory loss. *Malassezia furfur* - This yeast is responsible for **tinea versicolor**, characterized by **hypopigmented or hyperpigmented patches** that typically scale. - However, it does not cause **nerve damage** or accompanying sensory loss, and systemic symptoms like fever and malaise are not typical. *Pseudomonas aeruginosa* - This bacterium can cause various opportunistic infections, including skin infections like **ecthyma gangrenosum** or **folliculitis** in immunocompromised patients. - *Pseudomonas* infections are usually painful, often associated with a characteristic **grape-like odor**, and do not typically present with chronic, anesthetic, hypopigmented lesions. *Leishmania donovani* - This parasite causes **visceral leishmaniasis** (kala-azar), which presents with fever, weight loss, hepatosplenomegaly, and **hyperpigmentation of the skin** (darkening), not hypopigmentation. - It does not cause localized anesthetic skin lesions like those described in the patient.

Question 144: An investigator inoculates three different broths with one colony-forming unit of Escherichia coli. Broth A contains 100 μmol of lactose, broth B contains 100 μmol of glucose, and broth C contains both 100 μmol of lactose and 100 μmol of glucose. After 24 hours, the amounts of lactose, galactose, and glucose in the three broths are measured. The results of the experiment are shown: Lactose Galactose Glucose Broth A 43 μmol 11 μmol 9 μmol Broth B 0 μmol 0 μmol 39 μmol Broth C 94 μmol 1 μmol 66 μmol The observed results are most likely due to which of the following properties of broth A compared to broth C?

A. Absence of glucose preventing catabolite repression (Correct Answer)
B. Higher lactose concentration allowing increased metabolism
C. Lower pH favoring lactose utilization
D. Presence of additional nutrients enhancing growth
E. Reduced osmotic pressure facilitating enzyme activity

Explanation: ***Absence of glucose preventing catabolite repression*** - In Broth A, the absence of **glucose** means that **catabolite repression** does not occur, allowing the *E. coli* to immediately and more extensively utilize **lactose**. - This leads to a greater breakdown of lactose into galactose and glucose, and subsequent further metabolism of these sugars, evidenced by the lower final lactose concentration and higher galactose/glucose conversion than in Broth C. *Higher lactose concentration allowing increased metabolism* - All broths initially contain **100 μmol of lactose**, so there is no higher initial concentration in Broth A to explain the difference in metabolism. - The observed difference is in the *extent* of lactose utilization, not in the initial amount available. *Lower pH favoring lactose utilization* - The experiment does not provide any information about the **pH levels** in the broths. - While pH can affect enzyme activity, there is no basis in the given data to suggest a pH difference as the cause for varied lactose utilization. *Presence of additional nutrients enhancing growth* - All broths are inoculated with the same organism and the only specified difference is the sugar content (lactose, glucose, or both). - There is no mention of **additional nutrients** being present in Broth A compared to Broth C. *Reduced osmotic pressure facilitating enzyme activity* - The initial total solute concentration in Broth A (100 μmol lactose) is similar to Broth B (100 μmol glucose) and lower than Broth C (100 μmol lactose + 100 μmol glucose). - While **osmotic pressure** can affect bacterial growth, there is nothing in the data to suggest it is the primary factor explaining the difference in lactose metabolism between Broth A and Broth C.

Question 145: A 34-year-old, previously healthy woman is admitted to the hospital with abdominal pain and bloody diarrhea. She reports consuming undercooked beef a day before the onset of her symptoms. Her medical history is unremarkable. Vital signs include: blood pressure 100/70 mm Hg, pulse rate 70/min, respiratory rate 16/min, and temperature 36.6℃ (97.9℉). Physical examination shows paleness, face and leg edema, and abdominal tenderness in the lower right quadrant. Laboratory investigation shows the following findings: Erythrocytes 3 x 106/mm3 Hemoglobin 9.4 g/dL Hematocrit 0.45 (45%) Corrected reticulocyte count 5.5% Platelet count 18,000/mm3 Leukocytes 11,750/mm3 Total bilirubin 2.33 mg/dL (39.8 µmol/L) Direct bilirubin 0.2 mg/dL (3.4 µmol/L) Serum creatinine 4.5 mg/dL (397.8 µmol/L) Blood urea nitrogen 35.4 mg/dL (12.6 mmol/L) E. coli O157: H7 was identified in the patient’s stool. Which toxin is likely responsible for her symptoms?

A. Erythrogenic toxin
B. Shiga toxin (Correct Answer)
C. Enterotoxin type B
D. α-hemolysin
E. Exotoxin A

Explanation: ***Shiga toxin*** - The clinical presentation of **abdominal pain, bloody diarrhea, thrombocytopenia, microangiopathic hemolytic anemia** (indicated by low hemoglobin, elevated reticulocyte count, and elevated indirect bilirubin), and **acute kidney injury** (elevated creatinine and BUN with edema) strongly suggests **hemolytic uremic syndrome (HUS)**. - ***E. coli* O157:H7** is a common cause of HUS, and it produces **Shiga toxins** (Stx1 and Stx2, also known as verotoxins), which cause endothelial damage leading to the characteristic features of HUS. - Shiga toxins inhibit protein synthesis by cleaving ribosomal RNA, resulting in **endothelial cell damage, thrombotic microangiopathy**, and the classic triad of HUS. *Erythrogenic toxin* - This toxin is produced by **_Streptococcus pyogenes_** and is responsible for the rash seen in **scarlet fever**. - It is not associated with **_E. coli_ O157:H7 infection** or the development of HUS. *Enterotoxin type B* - **Enterotoxin type B** is typically associated with **_Staphylococcus aureus_** and acts as a **superantigen** causing food poisoning with symptoms like vomiting and diarrhea. - It does not cause HUS or the specific hematological and renal manifestations described in the patient. *α-hemolysin* - **Alpha-hemolysin** is a pore-forming cytolytic toxin produced by various bacteria, including **_Staphylococcus aureus_** and **_E. coli_**. - While some *E. coli* strains produce alpha-hemolysin, it is not the primary toxin responsible for HUS caused by **_E. coli_ O157:H7**. *Exotoxin A* - **Exotoxin A** is produced by **_Pseudomonas aeruginosa_** and inhibits protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2). - It is not produced by *E. coli* O157:H7 and is not associated with HUS or the clinical presentation in this patient.

Question 146: A 20-week-old infant is brought to an urgent care clinic by her mother because she has not been eating well for the past 2 days. The mother said her daughter has also been "floppy" since yesterday morning and has been unable to move or open her eyes since the afternoon of the same day. The child has recently started solid foods, like cereals sweetened with honey. There is no history of loose, watery stools. On examination, the child is lethargic with lax muscle tone. She does not have a fever or apparent respiratory distress. What is the most likely mode of transmission of the pathogen responsible for this patient’s condition?

A. Vertical transmission
B. Vector-borne disease
C. Direct contact
D. Contaminated food (Correct Answer)
E. Airborne transmission

Explanation: ***Contaminated food*** - The infant's symptoms of **lethargy**, widespread **flaccid paralysis** (floppy, unable to move or open eyes), and recent ingestion of **honey** (a known source of **Clostridium botulinum** spores) strongly suggest **infant botulism**. - **Infant botulism** is acquired through the ingestion of **Clostridium botulinum spores**, typically from environmental sources or contaminated food like honey, which then germinate in the infant's immature gut. *Vertical transmission* - **Vertical transmission** refers to the passage of a pathogen from mother to offspring during pregnancy, birth, or breastfeeding. - The clinical picture of **flaccid paralysis** and association with **honey ingestion** in this case does not align with typical vertically transmitted infections. *Vector-borne disease* - **Vector-borne diseases** are transmitted by an arthropod vector, such as mosquitoes or ticks. - There is no clinical or epidemiological evidence in the scenario to suggest an **arthropod vector** as the source of this infant's illness. *Direct contact* - Diseases transmitted by **direct contact** typically require close physical interaction with an infected individual or their body fluids. - The onset of **neurological symptoms** and the specific history of **honey ingestion** do not point to direct contact as the mode of transmission for botulism. *Airborne transmission* - **Airborne transmission** occurs when pathogens are spread through respiratory droplets or aerosols. - The symptoms of **flaccid paralysis** and the history of recent **honey ingestion** are not consistent with an airborne pathogen.

Question 147: A 34-year-old man presents with a 2-day history of loose stools, anorexia, malaise, and abdominal pain. He describes the pain as moderate, cramping in character, and diffusely localized to the periumbilical region. His past medical history is unremarkable. He works as a wildlife photographer and, 1 week ago, he was in the Yucatan peninsula capturing the flora and fauna for a magazine. The vital signs include blood pressure 120/60 mm Hg, heart rate 90/min, respiratory rate 18/min, and body temperature 38.0°C (100.4°F). Physical examination is unremarkable. Which of the following is a characteristic of the microorganism most likely responsible for this patient’s symptoms?

A. Disabling Gi alpha subunit
B. Production of lecithinase
C. Overactivation of guanylate cyclase (Correct Answer)
D. Inactivation of the 60S ribosomal subunit
E. Presynaptic vesicle dysregulation

Explanation: ***Overactivation of guanylate cyclase*** - The patient's symptoms (loose stools, abdominal cramping, recent travel to the Yucatan Peninsula) are highly suggestive of **traveler's diarrhea**, most commonly caused by **enterotoxigenic E. coli (ETEC)**. - ETEC produces a **heat-stable toxin (ST)** that binds to the **guanylate cyclase C receptor** on intestinal epithelial cells, leading to **increased intracellular cGMP** and subsequent **chloride and water secretion**. *Disabling Gi alpha subunit* - This mechanism is characteristic of **pertussis toxin** (from *Bordetella pertussis*), which **ADP-ribosylates and inactivates the Gi protein**, preventing inhibition of adenylate cyclase. - Pertussis toxin is associated with **whooping cough**, not gastrointestinal disease or traveler's diarrhea. - Note: **Cholera toxin** works via a different mechanism—it **activates Gs alpha subunit** to increase cAMP, causing severe watery diarrhea, but the clinical presentation here (mild symptoms, travel to endemic area) favors ETEC over cholera. *Production of lecithinase* - **Lecithinase (alpha-toxin)** is a characteristic virulence factor of **Clostridium perfringens**, causing gas gangrene and some food poisoning, not the watery diarrhea described here. - It acts as a phospholipase, disrupting cell membranes. *Inactivation of the 60S ribosomal subunit* - This mechanism is associated with **Shiga toxin** produced by **enterohemorrhagic E. coli (EHEC)** and **Shigella dysenteriae**. - These typically cause **bloody diarrhea** and **hemolytic uremic syndrome (HUS)**, which are not described in this patient. *Presynaptic vesicle dysregulation* - This mechanism is characteristic of **botulinum toxin** (from *Clostridium botulinum*), which **cleaves SNARE proteins** and prevents acetylcholine release, causing flaccid paralysis. - It is not involved in bacterial gastroenteritis causing diarrhea.

Question 148: A 40-year-old woman comes to the emergency department due to severe right flank pain, fever, chills, and decreased urine output. The vital signs include a temperature of 39.0°C (102.2°F), heart rate of 120/min, a regular breathing pattern, and blood pressure of 128/70 mm Hg. Cardiopulmonary auscultation is normal. In addition, tenderness is elicited by right lumbar percussion. After initiating intravenous antibiotics empirically, the condition of the patient improves significantly. However, a low urine output persists. The results of the ordered laboratory tests are as follows: Urine culture Proteus mirabilis, > 150,000 CFU/mL (normal range: < 100,000 CFU/mL to no bacterial growth in asymptomatic patients) Density 1.030; Leukocyte esterase (+); Nitrites (+) pH 7.8 (normal range: 4.5–8.0) C-reactive protein 60 mg/dL (normal range: 0–10 mg/dL) Serum creatinine 1.8 mg/dL (normal range: 0.6–1.2 mg/dL) BUN 40 mg/dL (normal range: 7–20 mg/dL) Plain abdominal film Complex renal calculus in the right kidney Which of the following is the most likely type of stone the patient has?

A. Cystine
B. Uric acid
C. Xanthine
D. Calcium oxalate
E. Struvite (Correct Answer)

Explanation: ***Struvite*** - The presence of **_Proteus mirabilis_** in the urine culture, combined with a **high urine pH (7.8)** and a **"complex renal calculus"** (often a **staghorn calculus**), strongly points to a **struvite stone**. - **_Proteus_** and other urease-producing bacteria hydrolyze urea to ammonia, creating an alkaline environment that favors the precipitation of **magnesium ammonium phosphate (struvite)**. *Cystine* - **Cystine stones** are associated with the genetic disorder **cystinuria** and often present at a younger age; they are uncommon and typically appear as hexagonal crystals. - The urine pH in cystinuria is usually acidic or neutral, contrasting with the **alkaline urine** seen in this patient. *Uric acid* - **Uric acid stones** are typically found in patients with **gout** or those with high purine intake, and they are usually seen in **acidic urine** environments. - They are **radiolucent** (not visible on plain X-ray), but the association with **_Proteus_ infection** and **alkaline urine** is absent. *Xanthine* - **Xanthine stones** are extremely rare and result from a deficiency of **xanthine oxidase** or treatment with **allopurinol** for conditions like gout. - The patient's presentation does not align with the typical causes or characteristics of **xanthine urolithiasis**. *Calcium oxalate* - **Calcium oxalate stones** are the most common type of kidney stone but are not typically associated with **urinary tract infections** (UTIs) caused by **urease-producing bacteria**. - While they can be complex, the presence of **_Proteus_** and **alkaline urine** strongly suggests an infectious **struvite stone**.

Question 149: A 35-year-old man comes to the emergency department with fever, chills, dyspnea, and a productive cough. His symptoms began suddenly 2 days ago. He was diagnosed with HIV 4 years ago and has been on triple antiretroviral therapy since then. He smokes one pack of cigarettes daily. He is 181 cm (5 ft 11 in) tall and weighs 70 kg (154 lb); BMI is 21.4 kg/m2. He lives in Illinois and works as a carpenter. His temperature is 38.8°C (101.8°F), pulse is 110/min, respirations are 24/min, and blood pressure is 105/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. Examinations reveals crackles over the right lower lung base. The remainder of the examination shows no abnormalities. Laboratory studies show: Hemoglobin 11.5 g/dL Leukocyte count 12,800/mm3 Segmented neutrophils 80% Eosinophils 1% Lymphocytes 17% Monocytes 2% CD4+ T-lymphocytes 520/mm3(N ≥ 500) Platelet count 258,000/mm3 Serum Na+ 137 mEq/L Cl- 102 mEq/L K+ 5.0 mEq/L HCO3- 22 mEq/L Glucose 92 mg/dL An x-ray of the chest shows a right lower-lobe infiltrate of the lung. Which of the following is the most likely causal organism?

A. Streptococcus pneumoniae (Correct Answer)
B. Legionella pneumophila
C. Pneumocystis jirovecii
D. Staphylococcus aureus
E. Cryptococcus neoformans

Explanation: ***Streptococcus pneumoniae*** - This patient presents with **fever, chills, productive cough, dyspnea, leukocytosis with neutrophilia, and a lobar infiltrate on chest X-ray**, which are classic signs of **community-acquired bacterial pneumonia**. - Although the patient is **HIV-positive**, his CD4+ count is >500/mm3 and he is on antiretroviral therapy, indicating relatively preserved immune function, making *S. pneumoniae* the most common cause of pneumonia even in HIV-infected individuals with controlled disease. *Legionella pneumophila* - While *Legionella* can cause pneumonia with fever and dyspnea, it is often associated with **gastrointestinal symptoms** (e.g., diarrhea) and **hyponatremia**, which are not present here. - Exposure to contaminated water sources is a common risk factor, and the lobar infiltrate is less typical than diffuse or patchy infiltrates. *Pneumocystis jirovecii* - *Pneumocystis pneumonia (PJP)* is typically seen in **HIV patients with severely suppressed immune systems (CD4+ count <200/mm3)**. - The patient's CD4+ count (520/mm3) is above this threshold, and PJP usually presents with diffuse interstitial infiltrates rather than a lobar infiltrate. *Staphylococcus aureus* - *S. aureus* pneumonia often occurs in the context of recent **influenza infection, intravenous drug use, or hospitalization**, or can present rapidly with **necrotizing pneumonia** or **empyema**. - While possible, the absence of these specific risk factors or severe features makes it less likely than *S. pneumoniae* in this specific presentation. *Cryptococcus neoformans* - *Cryptococcus neoformans* is an opportunistic fungus that typically causes **pulmonary or central nervous system infections**, especially in severely immunocompromised patients (CD4+ count usually <100/mm3). - Pulmonary cryptococcosis often manifests as **nodules or cavitary lesions**, or can be asymptomatic, which differs from the acute lobar pneumonia presented.

Question 150: A 62-year-old woman presents to the emergency department for evaluation of a spreading skin infection that began from an ulcer on her foot. The patient has type 2 diabetes mellitus that is poorly controlled. On examination, there is redness and erythema to the lower limb with skin breakdown along an extensive portion of the leg. The patient’s tissues separate readily from the fascial plane, prompting a diagnosis of necrotizing fasciitis. What is the exotoxin most likely associated with this patient’s presentation?

A. Streptococcal pyogenic exotoxin A
B. TSST-1
C. Diphtheria toxin
D. Exfoliative toxin
E. Streptococcal pyogenic exotoxin B (Correct Answer)

Explanation: ***Streptococcal pyogenic exotoxin B*** - **Streptococcal pyogenic exotoxin B** is a **cysteine protease** that directly degrades tissue, including collagen and fibronectin, leading to the rapid tissue destruction characteristic of **necrotizing fasciitis**. - This exotoxin is frequently associated with **Group A Streptococcus (GAS)** infections, a common cause of severe soft tissue infections, especially in immunocompromised individuals like diabetics. *Streptococcal pyogenic exotoxin A* - This exotoxin acts as a **superantigen**, primarily causing symptoms of **streptococcal toxic shock syndrome** (STSS), characterized by fever, rash, and organ failure. - While GAS can cause necrotizing fasciitis, Exotoxin A is more closely linked to toxic shock phenomena rather than direct tissue destruction. *TSST-1* - **Toxic Shock Syndrome Toxin-1 (TSST-1)** is produced by **Staphylococcus aureus** and is a classic cause of **staphylococcal toxic shock syndrome**. - It acts as a **superantigen** but is not directly responsible for the extensive tissue necrosis seen in necrotizing fasciitis caused by streptococci. *Diphtheria toxin* - **Diphtheria toxin**, produced by *Corynebacterium diphtheriae*, inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death. - It causes diphtheria, characterized by a **pseudomembrane** in the throat and myocarditis, not necrotizing fasciitis. *Exfoliative toxin* - **Exfoliative toxins A and B** are produced by **Staphylococcus aureus** and are responsible for **Staphylococcal Scalded Skin Syndrome (SSSS)**. - These toxins cause cleavage of desmoglein-1 in the epidermis, leading to widespread blistering and desquamation, not deep tissue necrosis.

Question 151: A previously healthy 13-year-old girl is brought to the physician by her parents because of a 2-day history of low-grade fever, headache, nausea, and a sore throat. Examination of the oral cavity shows enlarged, erythematous tonsils with exudates and palatal petechiae. There is cervical lymphadenopathy. Her parents agree to her participating in a study of microbial virulence factors. A culture of the girl's throat is obtained and an organism is cultivated. The physician finds that the isolated organism is able to withstand phagocytosis when placed in fresh blood. The most likely explanation for this finding is the expression of which of the following?

A. M Protein (Correct Answer)
B. Protein A
C. Hyaluronidase
D. IgA protease
E. Streptolysin O

Explanation: ***M Protein*** - **M protein** is a major **virulence factor** of *Streptococcus pyogenes*, the likely causative agent of the observed pharyngitis, allowing it to resist **phagocytosis** by inhibiting complement activation and binding to fibrinogen. - The clinical presentation of **erythematous tonsils with exudates**, **palatal petechiae**, and **cervical lymphadenopathy** is highly characteristic of streptococcal pharyngitis (strep throat). *Protein A* - **Protein A** is a virulence factor associated with *Staphylococcus aureus*, which binds to the **Fc region of IgG** antibodies, thereby preventing opsonization and phagocytosis. - *S. aureus* typically causes skin infections, abscesses, or food poisoning, which do not align with the given symptoms of pharyngitis and palatal petechiae. *Hyaluronidase* - **Hyaluronidase** is an enzyme produced by several bacteria, including *Streptococcus pyogenes* and *Staphylococcus aureus*, which breaks down **hyaluronic acid in connective tissue**, facilitating bacterial spread. - While it contributes to invasiveness, hyaluronidase itself does not directly protect bacteria from **phagocytosis** in fresh blood in the same manner as M protein. *IgA protease* - **IgA protease** is an enzyme produced by bacteria such as *Neisseria gonorrhoeae*, *Neisseria meningitidis*, and *Haemophilus influenzae*, which cleaves **secretory IgA** antibodies. - This enzyme helps bacteria colonize mucous membranes by inactivating superficial antibodies, but it does not directly prevent **phagocytosis** by immune cells in fresh blood. *Streptolysin O* - **Streptolysin O** is an **exotoxin** produced by *Streptococcus pyogenes* that causes **hemolysis** and is toxic to various host cells, including phagocytes. - While it contributes to tissue damage and can inhibit phagocyte function, it primarily acts by lysing cells rather than preventing the initial engulfment from occurring, as M protein does.

Question 152: A 55-year-old man presents with fever, chills, fatigue, cough, sore throat, and breathlessness for the past 7 days. He describes the cough as productive and says he is fatigued all the time. He says he is a farmer with daily contact with rabbits, horses, sheep, pigeons, and chickens and reports cleaning the barn 3 days before his symptoms started. The patient denies any history of tick bites. Past medical history is irrelevant. His temperature is 39.4°C (103.0°F), pulse is 110/min, and respirations are 26/min. On physical examination, there are decreased breath sounds on the right side. A large tender node is palpable in the right axilla. A chest radiograph reveals multiple homogenous opacities in the lower lobe of the right lung and a right-sided pleural effusions. Gram staining of a sputum sample is negative for any organism. Serology tests are negative. Which of the following is the most likely causative organism for this patient’s condition?

A. Mycoplasma pneumoniae
B. Staphylococcus aureus
C. Bacillus anthracis
D. Francisella tularensis (Correct Answer)
E. Yersinia pestis

Explanation: ***Francisella tularensis*** - This patient's symptoms (fever, chills, fatigue, cough, sore throat, breathlessness, unilateral axillary lymphadenopathy) combined with his occupation as a farmer and recent exposure to various animals (rabbits, sheep, pigeons, chickens) are highly suggestive of **tularemia**. - **Tularemia**, caused by **_Francisella tularensis_**, can manifest as pneumonic tularemia (given the lung opacities and pleural effusion) and can also present with prominent lymphadenopathy, especially if there's inoculation through skin (e.g., handling infected animals or their products). The negative Gram stain is also consistent, as it is a **fastidious gram-negative coccobacillus** often difficult to culture or visualize. *Mycoplasma pneumoniae* - While **_Mycoplasma pneumoniae_** can cause atypical pneumonia with respiratory symptoms, it typically does not present with pronounced **axillary lymphadenopathy** or a history of severe systemic illness and animal exposure as seen here. - The chest radiograph findings of **multiple homogenous opacities** are less typical for _Mycoplasma_ pneumonia, which often shows more diffuse or interstitial patterns. *Staphylococcus aureus* - **_Staphylococcus aureus_** can cause severe pneumonia, often necrotizing, with effusions, but it usually presents with a more acute onset, can be associated with other risk factors (e.g., intravenous drug use, influenza), and would typically be visible on **Gram stain** as gram-positive cocci in clusters. - While S. aureus can cause lymphadenitis, the overall clinical picture, including the specific exposure history and the **negative Gram stain**, makes it less likely than _Francisella tularensis_. *Bacillus anthracis* - **_Bacillus anthracis_** could be considered due to animal exposure, as it causes **anthrax**. However, inhalational anthrax typically presents with rapid progression to severe mediastinal widening and hemorrhagic pleural effusions on chest imaging, often leading to very high mortality. - The described lung opacities and the axillary lymphadenopathy, while possible with anthrax, do not fit as precisely as **tularemia** given the specific **farmer exposure** and symptoms. *Yersinia pestis* - **_Yersinia pestis_** causes **plague**, which can manifest as pneumonic plague or bubonic plague. Pneumonic plague presents with severe, rapidly progressive pneumonia. Bubonic plague is characterized by painful, swollen lymph nodes (**buboes**). - While the patient has lymphadenopathy and lung involvement, the history of exposure to various farm animals (especially rabbits, which are common carriers of tularemia) combined with the less hyperacute presentation compared to typical plague, makes **_Francisella tularensis_** a more fitting diagnosis.

Question 153: A 4-year-old boy presents with bloody diarrhea. The patient’s mother states that he was fine this morning, but around midday, she received a call from his daycare center stating that the patient had a single bloody bowel movement. His mother states that there have been no recent changes in his diet, although they did attend a barbecue over the weekend. The patient has no recent history of fever, chills, or similar symptoms in the past. No significant past medical history. Initial laboratory studies show an elevated white blood cell (WBC) count, anemia, and a blood urea nitrogen (BUN)/creatinine ratio of 40. Stool examination shows 3+ blood but no fecal leukocytes. Which of the following tests would be diagnostic for this patient’s most likely condition?

A. Polymerase chain reaction (PCR) for DNA sequences in stool
B. Test stool for C. difficile toxins
C. Gram stain for gull-winged, curved rods
D. Stool culture in sorbitol-MacConkey medium (Correct Answer)
E. Sigmoidoscopy

Explanation: ***Stool culture in sorbitol-MacConkey medium*** - This patient's symptoms (bloody diarrhea, recent barbecue) and lab findings (anemia, elevated BUN/creatinine ratio suggesting **hemolytic uremic syndrome** or HUS) are highly suggestive of **E. coli O157:H7 infection**. - **Sorbitol-MacConkey agar** is a selective medium used to differentiate non-sorbitol-fermenting E. coli O157:H7 from other E. coli strains, which typically ferment sorbitol. *Polymerase chain reaction (PCR) for DNA sequences in stool* - While **PCR** can detect bacterial DNA, it is generally used for specific targets like **toxins (e.g., Shiga toxin)** or for organisms difficult to culture. - A direct PCR for unspecified DNA sequences wouldn't be as specific or definitive as a targeted culture for *E. coli O157:H7* in this clinical context. *Test stool for C. difficile toxins* - **Clostridium difficile infection** typically presents with watery diarrhea, although it can cause bloody diarrhea in severe cases and often follows antibiotic use. - The patient's history of a barbecue and the specific clinical picture (risk for HUS) make *C. difficile* less likely than *E. coli O157:H7*. *Gram stain for gull-winged, curved rods* - **Gram stain for "gull-winged, curved rods"** is characteristic of **Campylobacter jejuni**, which can cause bloody diarrhea. - While *Campylobacter* is a possibility, the strong suspicion for **E. coli O157:H7** due to the HUS risk factors points towards specific culture methods as the initial diagnostic step. *Sigmoidoscopy* - **Sigmoidoscopy** is an invasive procedure generally reserved for cases where infectious etiologies haven't been identified or if there's suspicion of inflammatory bowel disease, severe colitis, or ischemic colitis. - It is not the initial diagnostic test for suspected acute infectious bloody diarrhea with a clear epidemiologic link.

Question 154: A 10-month-old boy is brought to the emergency department by his parents because he has a high fever and severe cough. His fever started 2 days ago and his parents are concerned as he is now listless and fatigued. He had a similar presentation 5 months ago and was diagnosed with pneumonia caused by Staphylococcus aureus. He has been experiencing intermittent diarrhea and skin abscesses since birth. The child had an uneventful birth and the child is otherwise developmentally normal. Analysis of this patient's sputum reveals acute angle branching fungi and a throat swab reveals a white plaque with germ tube forming yeast. Which of the following is most likely to be abnormal in this patient?

A. NADPH oxidase activity (Correct Answer)
B. Lysosomal trafficking
C. LFA-1 integrin binding
D. Thymus development
E. Autoimmune regulator function

Explanation: ***NADPH oxidase activity*** * The recurrent *Staphylococcus aureus* pneumonia, skin abscesses, and severe infections with unusual organisms like *acute angle branching fungi* (suggesting *Aspergillus*) and *germ tube-forming yeast* (*Candida*) are hallmarks of **Chronic Granulomatous Disease (CGD)**. * CGD is characterized by a defect in **NADPH oxidase**, which impairs the phagocytes' ability to produce reactive oxygen species (respiratory burst) to kill certain catalase-positive bacteria and fungi. *Lysosomal trafficking* * Defects in **lysosomal trafficking** are associated with disorders like **Chédiak-Higashi syndrome**, characterized by partial oculocutaneous albinism, recurrent pyogenic infections, and neurological abnormalities. * While recurrent infections are present, the specific combination of opportunistic fungal infections and *S. aureus* is less typical for Chédiak-Higashi, and the prominent albinism is not mentioned. *LFA-1 integrin binding* * Defective **LFA-1 integrin binding** is characteristic of **Leukocyte Adhesion Deficiency (LAD) type 1**, which manifests as recurrent bacterial infections, impaired wound healing, and delayed umbilical cord separation. * While recurrent bacterial infections are present, the prominent fungal infections and skin abscesses are more directly indicative of a phagocytic oxidative burst defect. *Thymus development* * Abnormal **thymus development** is associated with **T-cell immunodeficiencies** such as **DiGeorge syndrome** or **SCID**, leading to recurrent viral, fungal, and opportunistic infections. * While fungal infections might occur, the recurrent *S. aureus* infections and skin abscesses point more specifically to a **phagocytic defect** rather than primarily a T-cell deficiency. The child is also described as developmentally normal. *Autoimmune regulator function* * Dysfunctional **Autoimmune Regulator (AIRE) function** is typically seen in **Autoimmune Polyendocrine Syndrome Type 1 (APS-1)**, characterized by chronic mucocutaneous candidiasis (which aligns with the throat swab finding) and autoimmune endocrinopathies. * While candidiasis is present, the severe recurrent *Staphylococcus aureus* pneumonia, *Aspergillus* infection, and skin abscesses are not characteristic of APS-1, which primarily affects T-cell education in the thymus and leads to autoimmunity.

Question 155: A scientist is studying the process by which innate immune cells are able to respond to damage and pathogen infiltration. Specifically, she examines patients with an immunodeficiency where they are unable to respond to local infections. She notices that these patients do not produce pustulant fluid and do not have recruitment of immune cells in the first several hours of inflammation. Examining neutrophils within these patients reveals that they are able to slow their movement in a flow chamber by loosely attaching to purified vessel tissues. Subsequently, she shows that the neutrophils attach tightly to these vessel walls and move across the walls to the other side. Finally, when different levels of pathogenic proteins are placed on two sides of a purified vessel wall, the neutrophils from this patient do not exhibit a preference between the two sides. The step of neutrophil recruitment that is most likely defective in this patient involves which of the following mediators?

A. Nitric oxide
B. C5a (Correct Answer)
C. ICAM proteins
D. Selectins
E. Integrins

Explanation: ***C5a*** - The patient's neutrophils can **slow down**, **attach tightly**, and **transmigrate**, but they fail to exhibit **chemotaxis** (preferential movement) towards varying pathogenic protein levels. - **C5a** is a potent **chemoattractant** for neutrophils, guiding them to the site of infection and inflammation, and its defect would explain the lack of directed migration. *Nitric oxide* - **Nitric oxide** plays a role in **vasodilation** and has microbicidal activity, but it is not directly involved in guiding neutrophil migration. - Its defect would more likely manifest as impaired antimicrobial activity or altered vascular tone, not a failure of chemotaxis. *ICAM proteins* - **ICAM proteins** (specifically ICAM-1) on endothelial cells bind to **integrins** on neutrophils, mediating **tight adhesion** and **transmigration**. - Since the patient's neutrophils successfully perform tight adhesion and transmigration, their ICAM-integrin interaction is likely functional. *Selectins* - **Selectins** (E-selectin, P-selectin, L-selectin) are responsible for the initial **rolling** or "slowing down" of neutrophils along the vascular endothelium. - The patient's neutrophils are described as being able to "slow their movement" and loosely attach, indicating that selectin function is intact. *Integrins* - **Integrins** on neutrophils bind to ICAMs on endothelial cells, mediating **tight adhesion** and facilitating **transmigration**. - The patient's neutrophils are capable of both tight attachment and movement across vessel walls, suggesting their integrin function is normal.

Question 156: A 46-year-old man is brought to the emergency room by police after being found passed out on the sidewalk. He is intermittently alert and smells strongly of alcohol. He is unable to provide a history, but an electronic medical record search reveals that the patient has a history of alcohol abuse and was seen in the emergency room twice in the past year for alcohol intoxication. Further review of the medical record reveals that he works as a day laborer on a farm. His temperature is 98.8°F (37.1°C), blood pressure is 122/78 mmHg, pulse is 102/min, and respirations are 14/min. On examination, he is somnolent but arousable. He has vomitus on his shirt. He is given intravenous fluids and provided with supportive care. He vomits twice more and is discharged 6 hours later. However, 6 days after discharge, he presents to the emergency room again complaining of shortness of breath and fever. His temperature is 102°F (38.9°C), blood pressure is 100/58 mmHg, pulse is 116/min, and respirations are 24/min. The patient is actively coughing up foul-smelling purulent sputum. Which of the following is the most likely cause of this patient’s current symptoms?

A. A: Prevotella melaninogenica (Correct Answer)
B. D: Legionella pneumophila
C. E: Mycoplasma pneumoniae
D. C: Francisella tularensis
E. B: Coxiella burnetii

Explanation: **_Prevotella melaninogenica_** - This patient's clinical presentation, including a history of **alcohol abuse**, vomiting, and subsequent development of **foul-smelling purulent sputum**, is highly suggestive of **aspiration pneumonia** caused by oral anaerobic bacteria. - *Prevotella melaninogenica* is a common **anaerobic bacterium** found in the oral flora and is a frequent cause of aspiration pneumonia and lung abscesses, especially in patients with impaired consciousness due to alcohol. *Coxiella burnetii* - This is the causative agent of **Q fever**, which is often associated with contact with contaminated animal products or aerosols, particularly from livestock (e.g., cattle, sheep, goats). - While the patient works on a farm, **foul-smelling purulent sputum** and the history of aspiration are not typical features of Q fever. *Francisella tularensis* - This bacterium causes **tularemia**, an infection typically acquired through contact with infected animals, insect bites (ticks), or contaminated water. - The symptoms can include fever, chills, and pneumonia, but **foul-smelling sputum** is not a characteristic feature, and the mode of transmission doesn't align with the aspiration event. *Legionella pneumophila* - This bacterium causes **Legionnaires' disease**, a severe form of pneumonia often linked to contaminated water sources like air conditioning systems or hot tubs. - While it causes pneumonia with fever and cough, **foul-smelling sputum** is not characteristic, and the patient's history of alcohol-induced aspiration is a stronger clue. *Mycoplasma pneumoniae* - This causes **"walking pneumonia,"** which is typically a milder respiratory infection with symptoms like gradual onset of cough, fever, and headache. - It does not present with **foul-smelling purulent sputum** or a clinical picture consistent with aspiration pneumonia and its associated complications.

Question 157: An autopsy of a 75-year-old man reveals obliterating endarteritis of the vasa vasorum of the aorta. Which of the following investigations will most likely be positive in this patient?

A. Increased serum creatinine
B. Rapid plasma reagin (RPR) (Correct Answer)
C. Increased ketonuria
D. Perinuclear anti-neutrophil cytoplasmic antibody (p-ANCA)
E. Increased double-stranded (ds) DNA titer

Explanation: ***Rapid plasma reagin (RPR)*** - Obliterating endarteritis of the **vasa vasorum** of the aorta is a classic pathologic finding in **tertiary syphilis**. - **RPR** is a non-treponemal test used to screen for syphilis and is highly likely to be positive in active or previously treated syphilis. *Increased serum creatinine* - This would indicate **renal dysfunction**, which is not a direct or primary consequence of tertiary syphilis affecting the vasa vasorum. - While syphilis can affect the kidneys, it is not the most likely or direct correlate of the described aortic pathology. *Increased ketonuria* - **Ketonuria** is a sign of **ketoacidosis**, typically seen in uncontrolled diabetes or prolonged starvation. - This finding has no direct etiologic or pathophysiologic link to obliterating endarteritis of the aorta. *Perinuclear anti-neutrophil cytoplasmic antibody (p-ANCA)* - **p-ANCA** is associated with certain **vasculitides**, such as microscopic polyangiitis and Churg-Strauss syndrome. - The pathology described is specific to syphilis and not typically associated with ANCA-positive vasculitis. *Increased double-stranded (ds) DNA titer* - An elevated **dsDNA titer** is characteristic of **systemic lupus erythematosus (SLE)**. - SLE is an autoimmune disease with a different constellation of symptoms and vascular pathologies compared to syphilitic aortitis.

Question 158: A group of scientists is studying various methods of bacterial reproduction. They find out that bacteria also exchange genetic material via these reproductive processes. They are about to study one such method known as bacterial conjugation. Which of the following occurs through bacterial conjugation?

A. A single E. coli bacteria with resistance to gentamicin splits into two E. coli bacteria, both of which have resistance to gentamicin.
B. Two Pseudomonas aeruginosa bacteria with identical copies of a plasmid after sharing DNA through sex pili. (Correct Answer)
C. Helicobacter pylori producing a prophage-encoded toxin.
D. A multidrug-resistant Shigella species passing resistance factor R to a Streptococcus species.
E. A strain of MRSA acquiring the gene of capsulation from another encapsulated strain via DNA extraction.

Explanation: ***Two Pseudomonas aeruginosa bacteria with identical copies of a plasmid after sharing DNA through sex pili.*** - Bacterial conjugation involves the transfer of genetic material, typically plasmids, through direct cell-to-cell contact via a **Pilus** (often called a **sex pilus**). This process results in the recipient cell acquiring a copy of the plasmid from the donor cell. - The formation of identical copies of a plasmid in both donor and recipient *Pseudomonas aeruginosa* cells after conjugation illustrates the typical outcome of this genetic exchange, where the recipient becomes like the donor in terms of the plasmid's genetic content. *A single E. coli bacteria with resistance to gentamicin splits into two E. coli bacteria, both of which have resistance to gentamicin.* - This describes **binary fission**, a form of asexual reproduction, where a single bacterium divides into two identical daughter cells. It is not genetic exchange. - While both daughter cells will inherit the gentamicin resistance, this process does not involve the transfer of genetic material between two different cells. *Helicobacter pylori producing a prophage-encoded toxin.* - This scenario describes **lysogenic conversion**, where a bacterium (the *Helicobacter pylori*) incorporates bacteriophage DNA (prophage) into its genome, leading to the expression of new traits, such as toxin production. - Lysogenic conversion is a form of specialized transduction and does not involve direct cell-to-cell contact as seen in conjugation. *A multidrug-resistant Shigella species passing resistance factor R to a Streptococcus species.* - While this describes the transfer of a **resistance plasmid (R factor)**, it specifies transfer between two different genera of bacteria (*Shigella* and *Streptococcus*), which is plausible, but the crucial missing element for conjugation is the method of transfer. - Without specifying transfer via a **Pilus** or direct contact, it could also imply transduction or transformation, which are different mechanisms of genetic exchange. *A strain of MRSA acquiring the gene of capsulation from another encapsulated strain via DNA extraction.* - This describes **bacterial transformation**, where a bacterium takes up naked DNA from its environment. - The mention of **DNA extraction** explicitly points to the uptake of extracellular DNA, which is characteristic of transformation, not conjugation.

Question 159: Ten days after undergoing left hip replacement, a 73-year-old hospitalized man develops a fever, dyspnea, cough productive of yellow sputum, confusion, nausea, and diarrhea. Several patients in the hospital report similar symptoms. Physical examination shows decreased breath sounds on the left side and inspiratory crackles over the left lung. An x-ray of the chest shows opacities in the lower lobe of the left lung. Treatment with ampicillin does not improve his symptoms. Subsequent evaluation of the patient's urine detects a pathogen-specific antigen, confirming the diagnosis. Which of the following sources of infection is most likely responsible for this local disease outbreak?

A. Insufficient adherence to hand hygiene measures
B. Transmission via infectious respiratory droplets
C. Colonization of the air conditioning system (Correct Answer)
D. Entry through colonized intravenous catheters
E. Contamination of reheated hospital food

Explanation: ***Colonization of the air conditioning system*** - The patient's symptoms, including **fever, dyspnea, cough, confusion, nausea, and diarrhea**, along with **pulmonary infiltrates**, are consistent with **Legionnaires' disease**. - The detection of a **pathogen-specific antigen in the urine** confirms *Legionella pneumophila* infection, which commonly spreads through **aerosolized water from contaminated AC systems, cooling towers, or nebulizers**. The presence of a **local disease outbreak** in the hospital further points to this environmental source. *Insufficient adherence to hand hygiene measures* - While poor hand hygiene can lead to various **nosocomial infections**, it typically spreads pathogens like *Staphylococcus aureus* or *Clostridioides difficile*, which present differently and are not associated with **Legionnaires' disease**. - **Legionella** is primarily acquired by inhaling **contaminated aerosols**, not through direct or indirect contact via hands. *Transmission via infectious respiratory droplets* - Respiratory droplet transmission is characteristic of viruses like **influenza** or bacteria like *Streptococcus pneumoniae*, causing common respiratory infections. - *Legionella pneumophila* is **not spread person-to-person via respiratory droplets**, distinguishing it from these typical airborne pathogens. *Entry through colonized intravenous catheters* - **IV catheter-related infections** usually cause bloodstream infections, **sepsis**, or local site infections, not primarily **pneumonia with systemic symptoms** like nausea and diarrhea, nor an outbreak of **Legionnaires' disease**. - The pathogen identified via **urine antigen testing** would not be associated with IV catheter colonization. *Contamination of reheated hospital food* - Foodborne illnesses typically present with prominent **gastrointestinal symptoms** (e.g., severe vomiting, diarrhea) and may or may not include fever, but **pneumonia and respiratory symptoms** are not characteristic. - *Legionella* is a **waterborne bacterium** and is not typically transmitted via food.

Question 160: A group of scientists studied the effects of cytokines on effector cells, including leukocytes. They observed that interleukin-12 (IL-12) is secreted by antigen-presenting cells (APCs) in response to bacterial lipopolysaccharide. When a CD4+ T cell is exposed to this interleukin, which of the following responses will it have?

A. Responds to extracellular pathogens
B. Cell-mediated immune responses (Correct Answer)
C. Releases granzymes
D. Activate B cells
E. Secrete IL-4

Explanation: ***Cell-mediated immune responses*** - **IL-12** from antigen-presenting cells promotes the differentiation of **naïve CD4+ T cells** into **Th1 cells**. - **Th1 cells** are the primary drivers of **cell-mediated immunity**, producing cytokines like **IFN-γ** that activate macrophages and cytotoxic T cells to combat intracellular pathogens. *Responds to extracellular pathogens* - Responses to extracellular pathogens are primarily mediated by **Th2 cells** and **humoral immunity**. - **Th2 cells** are induced by cytokines like **IL-4** and are involved in allergic reactions and antiparasitic responses. *Releases granzymes* - **Granzymes** are released by **cytotoxic T lymphocytes (CTLs)** and **natural killer (NK) cells** to induce apoptosis in infected or cancerous cells. - While Th1 cells help activate CTLs, they do not directly release granzymes themselves. *Activate B cells* - **B cell activation** and antibody production are primarily driven by **Th2 cells** and **follicular helper T (Tfh) cells**. - Th1 cells are more involved in responses against intracellular pathogens, which typically do not involve direct B cell activation. *Secrete IL-4* - **IL-4** is the signature cytokine of **Th2 cells**, which are primarily involved in humoral immunity and allergic responses. - **IL-12** inhibits Th2 differentiation and promotes Th1 differentiation, so a CD4+ T cell exposed to IL-12 would not secrete IL-4.

Question 161: A 23-year-old man comes to the physician because of progressive pain, redness, and swelling of his left forearm. The symptoms began after he scratched his arm on a metal table 4 days ago. Examination of the left forearm shows a 2-cm, tender, erythematous, fluctuant lesion at the site of trauma. Incision and drainage of the lesion is performed and a small amount of thick, white liquid is expressed. Which of the following cytokines is involved in the recruitment of the primary cell type found in this liquid?

A. IL-2
B. IL-8 (Correct Answer)
C. IL-14
D. IL-11
E. IL-5

Explanation: ***IL-8*** - The patient's symptoms (pain, redness, swelling, tender, erythematous, fluctuant lesion containing thick, white liquid) are classic for a **bacterial abscess**, which is a collection of **neutrophils** (the primary cell type in pus). - **IL-8** (also known as CXCL8) is a potent **chemokine** that primarily functions to recruit **neutrophils** to sites of inflammation and infection. *IL-2* - **IL-2** is primarily associated with the **proliferation and differentiation of T cells**, particularly in the context of adaptive immunity. - While important in immune responses, it is not the primary cytokine responsible for direct neutrophil recruitment to an acute bacterial infection. *IL-14* - **IL-14** is less commonly discussed in the context of acute inflammatory responses and neutrophil recruitment. - Its main roles are related to B cell growth and differentiation, rather than directly attracting neutrophils. *IL-11* - **IL-11** is involved in various processes including hematopoiesis and mucosal protection, often promoting the growth of certain cell types. - It does not play a direct or prominent role in the immediate recruitment of neutrophils to a bacterial abscess. *IL-5* - **IL-5** is a cytokine predominantly associated with the **differentiation, maturation, and activation of eosinophils**. - It is critical in allergic reactions and parasitic infections, but not primarily involved in recruiting neutrophils to bacterial abscesses.

Question 162: A 16-year-old female presents to her pediatrician complaining of 2 weeks of fever and 1 week of swollen lumps in her left armpit. Upon examination of the left upper extremity, her physician notes the presence of a single papule which the patient claimed appeared one week ago. The patient started her first job at a pet store 2.5 weeks ago. Which of the following is the vector of transmission of the causative agent?

A. Cats (Correct Answer)
B. Rabbits
C. Animal urine
D. Parrots
E. Armadillos

Explanation: ***Cats*** - The combination of **fever**, **swollen lymph nodes** (lumps in the armpit), a **single papule**, and a recent history of working at a **pet store** strongly suggests **cat scratch disease**. - **Cat scratch disease** is caused by *Bartonella henselae*, which is primarily transmitted to humans through the **scratch or bite of a cat**, especially kittens. *Rabbits* - Rabbits are known vectors for diseases like **tularemia**, which can cause fever and swollen lymph nodes. - However, the typical presentation of tularemia often includes a more prominent **ulcerative lesion** at the site of inoculation, and the papule described is less characteristic. *Animal urine* - While animal urine can transmit diseases like **leptospirosis**, which can cause fever and various systemic symptoms, it typically does not present with a localized papule followed by regional lymphadenopathy in this manner. - Exposure to animal urine usually occurs through contact with contaminated water or soil, and the pet store context points more towards direct animal contact. *Parrots* - Parrots are associated with **psittacosis** (parrot fever), caused by *Chlamydia psittaci*. - Psittacosis primarily manifests as a **respiratory illness** (pneumonia) and does not typically present with a localized papule and regional lymphadenopathy. *Armadillos* - Armadillos are significant reservoirs for **Mycobacterium leprae**, the causative agent of **leprosy**. - Leprosy has a very long incubation period and presents with skin lesions, nerve damage, and sometimes lymphadenopathy, but a 2-week onset and the described acute symptoms are inconsistent with leprosy.

Question 163: A 3-year-old boy is brought to the physician for evaluation of a generalized, pruritic rash. The rash began during infancy and did not resolve despite initiating treatment with topical corticosteroids. Three months ago, he was treated for several asymptomatic soft tissue abscesses on his legs. He has been admitted to the hospital three times during the past two years for pneumonia. Physical examination shows a prominent forehead and a wide nasal bridge. Examination of the skin shows a diffuse eczematous rash and white plaques on the face, scalp, and shoulders. Laboratory studies show a leukocyte count of 6,000/mm3 with 25% eosinophils and a serum IgE concentration of 2,300 IU/mL (N = 0–380). Flow cytometry shows a deficiency of T helper 17 cells. The patient’s increased susceptibility to infection is most likely due to which of the following?

A. Impaired Ig class-switching in lymphocytes
B. Impaired DNA repair in lymphocytes
C. Impaired actin assembly in lymphocytes
D. Impaired chemotaxis of neutrophils
E. Impaired IL-17 secretion by Th17 cells (Correct Answer)

Explanation: ***Impaired IL-17 secretion by Th17 cells*** - The combination of **eczematous rash**, recurrent **skin abscesses**, recurrent **pneumonia**, **eosinophilia**, high **IgE levels**, and **dysmorphic facial features** (prominent forehead, wide nasal bridge) is characteristic of **hyper-IgE syndrome (Job's syndrome)**. - The question explicitly states **deficiency of T helper 17 cells** on flow cytometry, which directly explains the mechanism of infection susceptibility. - Hyper-IgE syndrome is caused by a defect in the **STAT3 signaling pathway**, which leads to **impaired Th17 cell differentiation and function**, resulting in reduced production of **IL-17** and **IL-22**, crucial cytokines for antifungal and antibacterial immunity, particularly against **Staphylococcus aureus** and **Candida**. *Impaired Ig class-switching in lymphocytes* - This is characteristic of **Hyper-IgM Syndrome**, where patients typically have normal or elevated IgM but very low levels of IgG, IgA, and IgE due to defects in **CD40-CD40L interaction** or AID enzyme. - While patients with Hyper-IgM syndrome also suffer from recurrent infections, their primary issue is usually with **opportunistic infections** and they don't typically present with the specific dermatological and facial features seen here. *Impaired DNA repair in lymphocytes* - This refers to conditions like **Ataxia-telangiectasia**, which involves defects in DNA repair mechanisms and leads to immunodeficiency, **ataxia**, and **telangiectasias**. - The clinical presentation of severe eczema, recurrent abscesses, and elevated IgE is not typical of **Ataxia-telangiectasia**. *Impaired actin assembly in lymphocytes* - This is characteristic of **Wiskott-Aldrich Syndrome**, which results from a defect in the **WASp gene** leading to impaired actin polymerization and cell motility. - Patients typically present with **thrombocytopenia** (leading to bleeding issues), **eczema**, and recurrent infections; however, the elevated IgE and specific infections (abscesses, pneumonia) are more suggestive of hyper-IgE syndrome than the general immunodeficiency of Wiskott-Aldrich. *Impaired chemotaxis of neutrophils* - This is a feature of **Leukocyte Adhesion Deficiency (LAD)** or **Chediak-Higashi syndrome**. - In LAD, neutrophils cannot extravasate to infection sites, leading to recurrent infections without pus formation and **delayed umbilical cord separation**, which is not described. In Chediak-Higashi, there are also giant lysosomes and partial albinism.

Question 164: A 27-year-old man presents to the emergency department with general weakness and fatigue. He states that he has not felt well for several days and can't take care of himself anymore due to fatigue. The patient has a past medical history of IV drug abuse, alcohol abuse, and multiple minor traumas associated with intoxication. His temperature is 104°F (40°C), blood pressure is 127/68 mmHg, pulse is 120/min, respirations are 19/min, and oxygen saturation is 98% on room air. A murmur is heard on cardiac exam. The patient is treated appropriately and transferred to the inpatient floor and recovers over the next several days. The patient has been unable to eat solids, though he has been drinking large amounts of juice. On day 5 of his stay, the patient states he feels much better. He is no longer febrile. His only concern is profuse and watery diarrhea and severe abdominal pain which he has been experiencing since yesterday. The patient is started on IV fluids and given oral fluid replacement as well. Which of the following is associated with the most likely underlying diagnosis?

A. Pseudomembranes of fibrin (Correct Answer)
B. Increased osmotic load
C. Schistocytes on peripheral smear
D. Ascitic fluid infection
E. Anti-Saccharomyces cerevisiae antibody positivity (ASCA)

Explanation: ***Pseudomembranes of fibrin*** - This patient's initial presentation with fever, new murmur, and IV drug abuse history suggests **infective endocarditis**, which would be treated with **broad-spectrum antibiotics**. - The subsequent development of profuse watery diarrhea and severe abdominal pain after several days points strongly to **Clostridioides difficile infection** (**CDI**), which is characterized by the formation of **pseudomembranes** composed of fibrin, mucus, and inflammatory cells in the colon. *Increased osmotic load* - **Osmotic diarrhea** is caused by the presence of unabsorbable solutes in the gut lumen, such as from **lactose intolerance** or **magnesium-containing antacids**. - While the patient is drinking juice which can contribute to osmotic load, his symptoms are more severe and associated with abdominal pain and follow antibiotic use, making **CDI** a more likely cause. *Schistocytes on peripheral smear* - **Schistocytes** are fragmented red blood cells seen in microangiopathic hemolytic anemias like **thrombotic thrombocytopenic purpura (TTP)** or **hemolytic uremic syndrome (HUS)**, often associated with toxin-mediated damage. - While some severe infections can lead to HUS, the clinical picture here is most consistent with **antibiotic-associated colitis** rather than a microangiopathic process. *Ascitic fluid infection* - **Ascitic fluid infection**, or **spontaneous bacterial peritonitis (SBP)**, occurs in patients with advanced liver disease and ascites. - This patient’s history does not mention liver disease or ascites, and his symptoms of profuse watery diarrhea are not typical of SBP, which usually presents with fever, abdominal pain, and worsening ascites. *Anti-Saccharomyces cerevisiae antibody positivity (ASCA)* - **ASCA** positivity is primarily associated with **Crohn's disease**, an inflammatory bowel disease. - While Crohn's disease can cause abdominal pain and diarrhea, it typically presents as a chronic condition with waxing and waning symptoms, and its acute onset post-antibiotic treatment makes it less likely than **CDI**.

Question 165: An investigator studying immune response administers a 0.5 mL intradermal injection of an autoclaved microorganism to a study volunteer. Four weeks later, there is a 12-mm, indurated, hypopigmented patch over the site of injection. Which of the following is the most likely explanation for the observed skin finding?

A. Increased lipid uptake by macrophages
B. Increased antibody production by B cells
C. Increased release of transforming growth factor beta
D. Increased activity of neutrophils
E. Increased activity of CD4+ Th1 cells (Correct Answer)

Explanation: ***Increased activity of CD4+ Th1 cells*** - The indurated patch, 4 weeks after intradermal injection, represents a **delayed-type hypersensitivity (DTH) reaction**, which is primarily mediated by **CD4+ Th1 cells**. - **CD4+ Th1 cells** recognize antigen presented by APCs, release cytokines like **IFN-γ**, and recruit macrophages, leading to inflammation and induration. *Increased lipid uptake by macrophages* - While macrophages are involved in DTH reactions, their primary role in this context is antigen presentation and cytokine release, not specifically increased lipid uptake. - Increased lipid uptake by macrophages is typically associated with conditions like **atherosclerosis** or **lipid storage disorders**, not a DTH skin reaction. *Increased antibody production by B cells* - **Antibody production by B cells** is characteristic of **humoral immunity** and immediate hypersensitivity reactions, not a delayed, indurated skin reaction. - The delayed onset and cellular nature of the reaction rule out a primary role for antibodies. *Increased release of transforming growth factor beta* - **TGF-β** is a cytokine primarily involved in **immune regulation**, **tissue repair**, and **fibrosis**, often having immunosuppressive effects. - While it can be involved in the resolution phase or chronic inflammation, it is not the primary driver of the initial indurated, hypopigmented patch in a DTH reaction. *Increased activity of neutrophils* - **Neutrophils** are primarily involved in **acute inflammatory responses** and bacterial infections, characterized by pus formation. - A delayed, indurated, and unresolved lesion strongly suggests a **cell-mediated response** rather than an acute neutrophilic infiltration.

Question 166: A bacterial isolate obtained from a hospitalized patient is found to be resistant to amikacin. The isolated bacteria most likely has which of the following characteristics?

A. Increased drug influx capacity
B. Enhanced ability to transfer acetyl groups (Correct Answer)
C. D-Ala to D-Lac mutation
D. Low-affinity penicillin binding protein
E. DNA topoisomerase II mutation

Explanation: ***Enhanced ability to transfer acetyl groups*** - **Amikacin** is an **aminoglycoside antibiotic** that can be inactivated by **bacterial enzymes** through acetylation, phosphorylation, or adenylylation. - Increased acetyltransferase activity leads to the **acetylation of amikacin**, rendering it unable to bind to the bacterial ribosome and inhibit protein synthesis. *Increased drug influx capacity* - This mechanism would lead to **increased sensitivity** to the drug, as more amikacin would enter the bacterial cell, not resistance. - **Aminoglycosides** typically enter bacteria via an **oxygen-dependent transport system**; resistance mechanisms often involve reduced influx or active efflux, not increased influx. *D-Ala to D-Lac mutation* - This mutation is characteristic of **vancomycin resistance** in bacteria, particularly in *Enterococcus* species. - It alters the **peptidoglycan precursor structure**, reducing vancomycin's binding affinity, and is unrelated to aminoglycoside resistance. *Low-affinity penicillin binding protein* - This mechanism is responsible for **methicillin resistance** (and broader beta-lactam resistance) in *Staphylococcus aureus* (MRSA) and other bacteria. - A **modified PBP** (e.g., PBP2a) prevents beta-lactam antibiotics from binding and inhibiting cell wall synthesis, which is not relevant to amikacin. *DNA topoisomerase II mutation* - Mutations in **DNA gyrase (topoisomerase II)** or **topoisomerase IV** confer resistance to **fluoroquinolone antibiotics**. - These mutations prevent fluoroquinolones from interfering with bacterial DNA replication and repair, unrelated to amikacin's mechanism of action.

Question 167: A 32-day-old boy is brought to the emergency department because he is found to be febrile and listless. He was born at home to a G1P1 mother without complications, and his mother has no past medical history. On presentation he is found to be febrile with a bulging tympanic membrane on otoscopic examination. Furthermore, he is found to have an abscess around his rectum that discharges a serosanguinous fluid. Finally, the remnants of the umbilical cord are found to be attached and necrotic. Which of the following processes is most likely abnormal in this patient?

A. Actin remodeling
B. Microtubule organization
C. Reactive oxygen species production
D. Neutrophil migration (Correct Answer)
E. Antibody class switching

Explanation: ***Neutrophil migration*** - The patient's presentation with multiple bacterial infections (otitis media, perirectal abscess, omphalitis) in a neonate strongly suggests a primary immunodeficiency affecting **phagocytic function**, such as **Leukocyte Adhesion Deficiency (LAD)**. - LAD is characterized by defective **neutrophil migration** to sites of infection due to issues with **CD18 integrins** or their ligands, leading to recurrent, severe bacterial infections and impaired wound healing. *Actin remodeling* - While actin remodeling is crucial for phagocytosis and cell motility, a primary defect in this specific process in neutrophils is less commonly the sole cause of such a widespread, severe infectious phenotype compared to defects in adhesion or granule function. - Disorders involving actin, like **Wiskott-Aldrich syndrome**, typically present with additional features like **thrombocytopenia** and **eczema**, which are not mentioned. *Microtubule organization* - Defects in microtubule organization can affect **neutrophil chemotaxis** and granule transport, as seen in **Chédiak-Higashi syndrome**. - However, Chédiak-Higashi syndrome is also associated with **partial oculocutaneous albinism** and **neurological symptoms**, which are absent in this case presentation. *Reactive oxygen species production* - Impaired **reactive oxygen species (ROS) production** is characteristic of **Chronic Granulomatous Disease (CGD)**. - CGD typically presents with recurrent infections, often by **catalase-positive organisms**, and granuloma formation, but the specific pattern of omphalitis and perirectal abscess without lung or liver involvement is less typical for CGD than for LAD. *Antibody class switching* - Defects in **antibody class switching** lead to conditions like **hyper-IgM syndrome**, primarily causing recurrent bacterial infections due to a lack of specific antibody types (IgG, IgA, IgE). - This typically involves **opsonization defects** and a different spectrum of infections, and doesn't directly explain the impaired wound healing or the characteristic presentation of omphalitis and perirectal abscess seen in this case.

Question 168: A 32-year-old woman is admitted to the hospital with headache, photophobia, vomiting without nausea, and fever, which have evolved over the last 12 hours. She was diagnosed with systemic lupus erythematosus at 30 years of age and is on immunosuppressive therapy, which includes oral methylprednisolone. She has received vaccinations—meningococcal and pneumococcal vaccination, as well as BCG. Her vital signs are as follows: blood pressure 125/70 mm Hg, heart rate 82/min, respiratory rate 15/min, and temperature 38.7°C (101.7°F). On examination, her GCS score is 15. Pulmonary, cardiac, and abdominal examinations are within normal limits. A neurologic examination does not reveal focal symptoms. Moderate neck stiffness and a positive Brudzinski’s sign are noted. Which of the following would you expect to note in a CSF sample?

A. Haemophilus influenzae growth in the CSF culture
B. Formation of a spiderweb clot in the collected CSF
C. Listeria monocytogenes growth in the CSF culture (Correct Answer)
D. Decrease in CSF protein level
E. Lymphocytic pleocytosis

Explanation: ***Listeria monocytogenes growth in the CSF culture*** - Patients with **systemic lupus erythematosus (SLE)** on **immunosuppressive therapy** (like methylprednisolone) are at increased risk for opportunistic infections, including **Listeria monocytogenes meningitis**. - *Listeria* meningitis commonly presents with **fever**, **headache**, and **neck stiffness**, even in patients who have received typical meningitis vaccinations. *Haemophilus influenzae growth is the CSF culture* - While *Haemophilus influenzae* can cause meningitis, the patient's history of **meningococcal and pneumococcal vaccination** makes this less likely, and *Listeria* is a more common pathogen in immunosuppressed adults. - The patient's age (32 years) also makes *Haemophilus influenzae* meningitis less common, as it primarily affects very young children or the elderly. *Formation of a spiderweb clot in the collected CSF* - A **spiderweb clot** in CSF is characteristic of **tuberculous meningitis**, which is typically a more chronic illness with a different clinical presentation (e.g., more insidious onset). - Although immunosuppression can predispose to tuberculosis, the acute onset (12 hours) and specific clinical picture do not fit classic tuberculous meningitis. *Decrease in CSF protein level* - A **decrease in CSF protein level** is generally *not* expected in bacterial meningitis; instead, **elevated protein levels** are a hallmark due to increased permeability of the blood-brain barrier and bacterial proteins. - Reduced CSF protein is a non-specific finding and not indicative of active bacterial infection. *Lymphocytic pleocytosis* - **Lymphocytic pleocytosis** is more characteristic of **viral meningitis** or certain chronic bacterial infections like **tuberculous meningitis** or **fungal meningitis**. - **Bacterial meningitis**, especially from pathogens like *Listeria*, typically causes **neutrophilic pleocytosis**.

Question 169: A 51-year-old man comes to the physician for 2 months of intermittent low-grade fever, malaise, and joint pain. He has a history of recurrent dental abscesses requiring drainage but has otherwise been healthy. His temperature is 38.3°C (100.9°F) and pulse is 112/min. Physical examination shows a new holosystolic murmur in the left midclavicular line that radiates to the axilla. There are linear reddish-brown lesions underneath the nail beds and tender violaceous nodules on the bilateral thenar eminences. Two sets of blood cultures grow Streptococcus mutans. A transthoracic echocardiogram shows moderate regurgitation of the mitral valve. Which of the following mechanisms is most likely directly involved in the pathogenesis of this patient's valvular condition?

A. Coagulative necrosis
B. Leaflet calcification and fibrosis
C. Antibody cross-reaction
D. Fibrin clot formation (Correct Answer)
E. Sterile platelet thrombi deposition

Explanation: ***Fibrin clot formation*** - This patient has **infective endocarditis**, characterized by **vegetations** on the heart valves. These vegetations are composed of platelets, fibrin, microorganisms, and inflammatory cells. - The formation of these vegetations involves the deposition of **fibrin and platelets** on damaged endothelial surfaces, which then become a nidus for bacterial colonization, specifically *Streptococcus mutans* in this case. *Coagulative necrosis* - **Coagulative necrosis** is a type of cell death typically seen in ischemic conditions (e.g., myocardial infarction, renal infarcts) where the cell architecture is preserved for a period. - It is not the primary mechanism for valvular damage and vegetation formation in **infective endocarditis**. *Leaflet calcification and fibrosis* - **Calcification and fibrosis** of valve leaflets are characteristic features of chronic degenerative valvular diseases, such as **aortic stenosis** in older age or chronic rheumatic heart disease. - While chronic inflammation can lead to fibrosis, the acute pathophysiology of infective endocarditis is dominated by vegetation formation and destruction rather than primary calcification. *Antibody cross-reaction* - **Antibody cross-reaction** is the mechanism responsible for **rheumatic fever**, where antibodies against streptococcal M proteins cross-react with cardiac tissue antigens (molecular mimicry), leading to valvular damage. - Although *Streptococcus* is involved, the signs and symptoms (new murmur, fever, splinter hemorrhages, Osler nodes, and positive blood cultures) are classic for **infective endocarditis**, not acute rheumatic fever. *Sterile platelet thrombi deposition* - **Sterile platelet thrombi deposition** occurs in **nonbacterial thrombotic endocarditis (NBTE)**, also known as marantic endocarditis, often associated with hypercoagulable states or malignancy. - While it involves platelet deposition, the key difference here is the presence of **positive blood cultures** with *Streptococcus mutans*, indicating an active infection, not a sterile process.

Question 170: 5 days after receiving chemotherapy for ovarian cancer, a 74-year-old woman comes to the physician for a follow-up examination. She feels well and has no complaints. Her leukocyte count is 3,500/mm3 (11% neutrophils and 89% lymphocytes). This patient's profound granulocytopenia is most likely to predispose her to infection with which of the following organisms?

A. Pseudomonas aeruginosa (Correct Answer)
B. Staphylococcus aureus
C. Candida albicans
D. Cytomegalovirus
E. Pneumocystis jirovecii

Explanation: ***Pseudomonas aeruginosa*** - Profound **granulocytopenia**, especially after chemotherapy, significantly increases the risk of gram-negative bacterial infections, with **Pseudomonas aeruginosa** being a common and severe pathogen due to its opportunistic nature. - Granulocytes are crucial for phagocytosing and destroying bacteria, making their deficiency a key factor in susceptibility to aggressive bacterial infections. *Staphylococcus aureus* - While *S. aureus* can cause severe infections in immunocompromised individuals, it is more commonly associated with compromised **skin barriers** or indwelling devices, rather than profound granulocytopenia being the primary predisposing factor compared to gram-negative rod infections. - Though it can cause life-threatening infections, **granulocytopenia** significantly elevates the risk for gram-negative bacteria over gram-positive cocci in the immediate post-chemotherapy period. *Candida albicans* - **Candida albicans** primarily causes opportunistic fungal infections, often in individuals with impaired **cell-mediated immunity** or prolonged broad-spectrum antibiotic use that disrupts normal flora. - While granulocytopenia can increase susceptibility to fungal infections, the immediate risk for overwhelming bacterial infections, particularly gram-negatives like *Pseudomonas*, is generally higher in the acute phase of chemotherapy-induced neutropenia. *Cytomegalovirus* - **Cytomegalovirus (CMV)** infection is a **viral opportunistic infection** seen in severely immunocompromised patients, particularly those with impaired **T-cell function**, such as transplant recipients or AIDS patients. - While chemotherapy affects overall immunity, acute granulocytopenia is not the primary predisposing factor for CMV reactivation or infection compared to bacterial or fungal pathogens. *Pneumocystis jirovecii* - **Pneumocystis jirovecii pneumonia** is a classic opportunistic infection in patients with profound **defects in T-cell mediated immunity**, such as HIV/AIDS patients or those on specific immunosuppressants. - Although general immunocompromise from chemotherapy can increase risk, **granulocytopenia** itself is not the most direct predisposing factor for *Pneumocystis* infection.

Question 171: A 43-year-old construction worker presents to the emergency department two hours after sustaining a deep laceration to his left forearm by a piece of soiled and rusted sheet metal. His vital signs are stable, there is no active bleeding, his pain is well controlled, and a hand surgeon has been notified about damage to his forearm tendons. He does not recall receiving any vaccinations in the last 30 years and does not know if he was vaccinated as a child. What is the appropriate post-exposure prophylaxis?

A. IV metronidazole only
B. Tetanus vaccine + immunoglobulin (Correct Answer)
C. Anthrax vaccine
D. Tetanus immunoglobulin only
E. Tetanus vaccine booster only

Explanation: ***Tetanus vaccine + immunoglobulin*** - For **contaminated wounds** in patients with uncertain or incomplete immunization histories, both **tetanus toxoid vaccine** and **tetanus immunoglobulin (TIG)** are indicated. The vaccine provides active immunity, while TIG offers immediate passive immunity. - The patient's injury from soiled and rusted sheet metal, combined with his unknown vaccination status, places him at **high risk for tetanus** infection. *IV metronidazole only* - **Metronidazole** is an antibiotic effective against anaerobic bacteria but does not provide adequate **tetanus prophylaxis** on its own. - It would not prevent tetanus or offer active immunity, which is crucial for long-term protection. *Anthrax vaccine* - **Anthrax vaccine** is used for protection against *Bacillus anthracis* and is not indicated for **wound prophylaxis** from a rusty metal injury. - This vaccine is typically given to individuals with occupational exposure or in cases of bioterrorism. *Tetanus immunoglobulin only* - While **tetanus immunoglobulin** provides immediate passive immunity, it does not induce **active immunity** for future protection. - It is crucial to also administer the **tetanus vaccine** to stimulate the patient's immune system to produce antibodies. *Tetanus vaccine booster only* - A **tetanus vaccine booster** is appropriate for patients with a known history of vaccination but who haven't received a booster within the last 5-10 years, depending on the wound type. - In this case, the patient has an unknown vaccination history, making passive immunity with **TIG** also necessary.

Question 172: A 22-year-old woman seeks evaluation at a local walk-in clinic for severe lower abdominal pain, vaginal discharge, and painful intercourse for the last couple of weeks. Her last day of menstruation was 1 week ago, and since then the pain has worsened. She is an out-of-town college student engaged in an open relationship with a fellow classmate and another partner from her hometown. Additional concerns include painful micturition and a low-grade fever for the same duration. The physical examination reveals a heart rate of 120/min, respiratory rate of 24/min, and temperature of 38.6°C (101.5°F). The pelvic examination shows an erythematous cervix with a mucopurulent exudate. The cervix bleeds when manipulated with a swab and is extremely tender with movement. Based on the clinical findings, which of the following agents is the most likely cause of her condition?

A. Chlamydia trachomatis (Correct Answer)
B. Mycoplasma genitalium
C. Mycobacterium tuberculosis
D. Neisseria gonorrhoeae
E. Streptococcus agalactiae

Explanation: ***Chlamydia trachomatis*** - This patient presents with classic **pelvic inflammatory disease (PID)** characterized by **severe lower abdominal pain**, **mucopurulent cervicitis**, **cervical motion tenderness**, and **fever**. - *Chlamydia trachomatis* is the **most common causative organism** of PID, accounting for approximately 50% of cases. - Her **high-risk sexual behavior** (multiple partners, open relationship) significantly increases STI risk. - While both *Chlamydia* and *Neisseria gonorrhoeae* can cause severe PID with identical presentations, **statistical likelihood** makes *Chlamydia* the most likely single causative agent. - Clinical presentation alone cannot reliably distinguish between these two organisms; **both require empiric coverage** in PID treatment. *Neisseria gonorrhoeae* - This is the **second most common** cause of PID (30-40% of cases) and frequently **co-exists** with *Chlamydia*. - The clinical presentation described (fever, cervicitis, cervical motion tenderness) can occur with gonorrhea, but these features are **not specific** enough to distinguish it from chlamydial PID. - When the question asks for "most likely" without specific diagnostic findings (Gram stain, culture), the **higher prevalence** of *Chlamydia* makes it the better answer. *Mycoplasma genitalium* - An **emerging pathogen** associated with cervicitis, urethritis, and PID. - While it can cause symptoms similar to those described, it accounts for a **much smaller proportion** of PID cases compared to *Chlamydia* and *Neisseria*. - Often presents with milder or chronic symptoms, though acute presentations can occur. *Mycobacterium tuberculosis* - **Genital tuberculosis** is rare in developed countries and typically presents with **chronic, insidious symptoms** such as infertility, menstrual irregularities, and chronic pelvic pain. - Would **not** cause acute mucopurulent cervicitis with cervical friability. - More common in endemic areas and immunocompromised patients. *Streptococcus agalactiae* - Group B Streptococcus is a common **vaginal colonizer** and causes infections primarily in **neonates** and **postpartum women**. - **Not a typical pathogen** for acute PID in non-pregnant, sexually active young women. - Does not commonly cause the mucopurulent cervicitis seen in this case.

Question 173: A 25-year-old woman presents to the ED with a diffuse, erythematous rash in the setting of nausea, vomiting, and fever for 2 days. Physical exam reveals a soaked tampon in her vagina. Blood cultures are negative. The likely cause of this patient's disease binds to which molecule on T cells?

A. Gamma chain of the IL-2 receptor
B. CD3
C. Variable beta portion of the T-cell receptor (Correct Answer)
D. Fas ligand
E. CD40 ligand

Explanation: ***Variable beta portion of the T-cell receptor*** - This patient's presentation with diffuse rash, fever, vomiting, and a retained tampon suggests **toxic shock syndrome (TSS)**, typically caused by toxins like TSST-1 from *Staphylococcus aureus*. - **Superantigens** like TSST-1 bind directly to the **variable beta portion of the T-cell receptor (TCR Vβ)** and the major histocompatibility complex (MHC) class II molecules, bypassing normal antigen presentation. This leads to massive, non-specific T-cell activation and cytokine storm. *Gamma chain of the IL-2 receptor* - The **gamma chain of the IL-2 receptor** is a common component of several cytokine receptors (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21) and is crucial for T-cell proliferation and differentiation. - While T-cells are activated in TSS and produce IL-2, the toxin does not directly bind to the IL-2 receptor to exert its primary effect. *CD3* - **CD3** is a multi-protein complex associated with the **T-cell receptor (TCR)** and is essential for signal transduction upon antigen binding. - Superantigens do not directly bind to CD3; their primary binding sites are the TCR Vβ region and MHC class II. *Fas ligand* - **Fas ligand (FasL)** is a transmembrane protein on immune cells that binds to Fas (CD95) on target cells, inducing **apoptosis**. - While apoptosis can occur in severe infections, superantigens primarily cause massive T-cell activation, not direct binding to FasL. *CD40 ligand* - **CD40 ligand (CD40L)**, found on activated T-cells, interacts with CD40 on B-cells and other antigen-presenting cells to provide co-stimulation necessary for B-cell activation, class switching, and memory cell formation. - Superantigens do not directly bind to CD40L; their mechanism involves non-specific T-cell activation, which can indirectly affect B-cell responses.

Question 174: A 34-year-old woman with a seizure disorder comes to the physician because of fever, fatigue, and a productive cough with foul-smelling sputum for 2 weeks. Her temperature is 38.3°C (100.9°F). Physical examination shows dullness to percussion over the right lung fields. An x-ray of the chest shows a cavitary infiltrate with an air-fluid level in the right lower lobe of the lung. Cultures of an aspirate of the infiltrate grow Peptostreptococcus and Prevotella species. Which of the following is the most likely predisposing factor for this patient's condition?

A. Periodontal infection (Correct Answer)
B. Recent hospitalization
C. Crowded housing situation
D. Contaminated air conditioning system
E. Intravenous drug use

Explanation: ***Periodontal infection*** - The presence of **Peptostreptococcus** and **Prevotella species** (anaerobic bacteria) in a lung abscess is highly suggestive of aspiration of oral flora, often linked to poor dental hygiene or **periodontal disease**. - The patient's **seizure disorder** increases the risk for aspiration events during which oral contents, including these anaerobic bacteria, can be inhaled into the lungs. *Recent hospitalization* - While hospitalization can expose patients to nosocomial infections, it is less likely to specifically predispose to a lung abscess caused by **oral anaerobes** unless associated with other risk factors like aspiration. - Common hospital-acquired pneumonias are caused by organisms like *Pseudomonas aeruginosa* or *Staphylococcus aureus*, not typically the anaerobic flora found here. *Crowded housing situation* - A crowded housing situation is a risk factor for droplet-borne infections such as tuberculosis or influenza, but not typically for a **lung abscess** caused by **anaerobic bacteria** unique to the oral cavity. - This environmental factor does not directly explain the specific microbiology or the cavitary lesion. *Contaminated air conditioning system* - Contaminated air conditioning systems are most notoriously associated with **Legionnaires' disease**, caused by *Legionella pneumophila*. - This bacterium does not cause lung abscesses with anaerobic flora like *Peptostreptococcus* or *Prevotella*, and the clinical presentation would be different. *Intravenous drug use* - Intravenous drug use is a major risk factor for **septic emboli** leading to lung abscesses, often caused by **Staphylococcus aureus** and typically presenting as multiple peripheral lesions. - This patient's specific microbiology (oral anaerobes) and the single cavitating lesion are not characteristic of drug-use related septic emboli.

Question 175: A 61-year-old man is brought to the emergency department because of a 2-day history of fever, chills, and headache. He frequently has headaches, for which he takes aspirin, but says that this headache is more intense. His wife claims that he has also not been responding right away to her. He has a 20-year history of hypertension and poorly controlled type 2 diabetes mellitus. His current medications include metformin and lisinopril. He has received all recommended childhood vaccines. His temperature is 39°C (102.2F°), pulse is 100/min, and blood pressure is 150/80 mm Hg. He is lethargic but oriented to person, place, and time. Examination shows severe neck rigidity with limited active and passive range of motion. Blood cultures are obtained and a lumbar puncture is performed. Which of the following is the most likely causal organism?

A. Streptococcus agalactiae
B. Staphylococcus aureus
C. Escherichia coli
D. Streptococcus pneumoniae (Correct Answer)
E. Neisseria meningitidis

Explanation: ***Streptococcus pneumoniae*** - This patient presents with classic symptoms of **bacterial meningitis** (fever, severe headache, neck rigidity, altered mental status) and is in an older age group with comorbidities (diabetes, hypertension). - *Streptococcus pneumoniae* is the **most common cause of bacterial meningitis in adults**, especially in those over 60 years old or with underlying medical conditions, despite prior vaccinations, as vaccine efficacy can wane or cover specific serotypes. *Streptococcus agalactiae* - *Streptococcus agalactiae* (**Group B Streptococcus**) is primarily known as a cause of **neonatal meningitis** and sepsis. - While it can cause disease in immunocompromised adults, it is a less common cause of meningitis in a 61-year-old with this specific presentation compared to *S. pneumoniae*. *Staphylococcus aureus* - *Staphylococcus aureus* meningitis typically occurs in the setting of **neurosurgery**, **trauma with dural breach**, or **endocarditis** with septic emboli, none of which are suggested here. - While possible, it is not as common a cause of community-acquired bacterial meningitis in this demographic as *S. pneumoniae*. *Escherichia coli* - *Escherichia coli* is a frequent cause of meningitis in **neonates** and sometimes in the **elderly with compromised immune systems** or following urinary tract infections. - While the patient is elderly and has diabetes, *S. pneumoniae* is a more likely cause given the general presentation of community-acquired meningitis in this age group. *Neisseria meningitidis* - *Neisseria meningitidis* meningitis is common in **children and young adults**, often presenting with a **petechial or purpuric rash**, which is not described in this patient. - While this patient received childhood vaccines, the typical age group for meningococcal disease makes *S. pneumoniae* a more probable pathogen for his current presentation.

Question 176: A 10-year-old boy is brought to the pediatric clinic because of a sore throat of 1-week duration. He also has a cough and fever. He has pain when swallowing and sometimes water regurgitates from his nose when drinking. He was diagnosed with acute tonsillitis by his primary care physician 1 month ago, for which he received a week-long course of amoxicillin. His immunization status is unknown as he recently moved to the US from Asia. On examination, he is alert and oriented to time, place, and person. On inspection of his oral cavity, an edematous tongue with a grey-white membrane on the soft palate and tonsils is noted. The neck is diffusely swollen with bilateral tender cervical lymphadenopathy. Which of the following is the cause of this patient’s condition and could have been prevented through vaccinations in childhood?

A. Candida albicans
B. Corynebacterium diphtheriae (Correct Answer)
C. Streptococcus pyogenes
D. Haemophilus influenzae b
E. Epstein Barr virus

Explanation: ***Corynebacterium diphtheriae*** - The presence of an **edematous tongue** with a **grey-white membrane** on the soft palate and tonsils, along with sore throat, fever, and difficulty swallowing (suggesting **palatal paralysis** secondary to diphtheria toxin), is highly characteristic of **diphtheria**. - Diphtheria is caused by **_Corynebacterium diphtheriae_** and is preventable by childhood vaccination (the DTaP vaccine). *Candida albicans* - **_Candida albicans_** causes **oral thrush** (candidiasis), which presents with white plaques on the oral mucosa and tongue, particularly in immunocompromised individuals or after antibiotic use. - However, these plaques are **easily removable** (revealing red, inflamed mucosa underneath), unlike the **firmly adherent grey-white pseudomembrane** of diphtheria; there is **no vaccine** against _Candida_. *Streptococcus pyogenes* - **_Streptococcus pyogenes_** causes **streptococcal pharyngitis** (strep throat), which typically presents with a sore throat, fever, and sometimes exudates on the tonsils, but it does **not form a pseudomembrane** with these specific characteristics (grey-white, firmly adherent). - While pharyngitis caused by _Streptococcus pyogenes_ can lead to complications, there is **no vaccine** against it. *Haemophilus influenzae b* - Prior to vaccination, **_Haemophilus influenzae_ type b (Hib)** was a major cause of bacterial meningitis, epiglottitis, and other invasive infections in children. - While it can cause respiratory infections, the classic presentation of **diphtheria with a pseudomembrane** and palatal paralysis is not typical for Hib infection; the Hib vaccine primarily targets invasive diseases. *Epstein Barr virus* - **Epstein-Barr virus (EBV)** causes **infectious mononucleosis**, which can present with a sore throat, fever, fatigue, and lymphadenopathy, sometimes with exudates on the tonsils. - However, it **does not typically form a firmly adherent grey-white pseudomembrane** with associated airway compromise or palatal paralysis; there is currently **no vaccine** for EBV.

Question 177: A 25-year-old patient comes to the physician with complaints of dysuria and white urethral discharge. He is sexually active with 4 partners and does not use condoms. The physician is concerned for a sexually transmitted infection and decides to analyze the nucleic acid sequences present in the discharge to aid in diagnosis via DNA amplification. Which of the following is responsible for the creation of the nucleic acid copies during the elongation phase of the technique most likely used in this case?

A. Nucleotide sequence of the target gene
B. Amino acid sequence of the target gene
C. DNA primers
D. Heat-resistant DNA polymerase (Correct Answer)
E. Heat-sensitive DNA polymerase

Explanation: ***Heat-resistant DNA polymerase*** - The process described is most likely **Polymerase Chain Reaction (PCR)**, a rapid and sensitive test for **nucleic acid amplification**. - **Heat-resistant DNA polymerase**, such as Taq polymerase, is crucial for synthesizing new DNA strands during the **elongation phase** at elevated temperatures. *Nucleotide sequence of the target gene* - The **nucleotide sequence of the target gene** is the template that is amplified, not the enzyme responsible for creating copies. - While essential for amplification, it doesn't directly catalyze the addition of nucleotides. *Amino acid sequence of the target gene* - The **amino acid sequence** is irrelevant to nucleic acid amplification as PCR targets DNA or RNA, not proteins. - This option reflects a misunderstanding of the molecular biology principles behind gene amplification techniques. *DNA primers* - **DNA primers** are short, synthetic oligonucleotides that define the start and end points of the DNA segment to be amplified. - They bind to the target DNA and provide a starting point for DNA polymerase but do not synthesize the new strands themselves. *Heat-sensitive DNA polymerase* - A **heat-sensitive DNA polymerase** would be denatured and inactivated during the high-temperature denaturation steps of PCR. - This would prevent the exponential amplification of the target DNA, making the technique ineffective.

Question 178: A 32-year-old man comes to the physician because of low-grade fever and progressive painful lumps in his right groin for 6 days. The lumps have been discharging purulent fluid since the evening of the previous day. He had a shallow, painless lesion on his penis 3 weeks ago, but was too embarrassed to seek medical attention; it has resolved in the meantime. There is no personal or family history of serious illness. He has smoked one pack of cigarettes daily for 12 years. He is sexually active with multiple male partners and uses condoms inconsistently. His temperature is 38.0°C (100.4°F). Examination of his groin shows multiple masses discharging pus. The remainder of the examination shows no abnormalities. Which of the following is the most likely causal organism?

A. Herpes simplex virus 2
B. Klebsiella granulomatis
C. Chlamydia trachomatis (Correct Answer)
D. Yersinia pestis
E. Haemophilus ducreyi

Explanation: ***Chlamydia trachomatis*** - The presentation of **painless genital ulcers** followed by painful inguinal lymphadenopathy that progresses to **suppurative buboes** (lumps discharging purulent fluid) is classic for **lymphogranuloma venereum (LGV)**, which is caused by specific serovars of *Chlamydia trachomatis* (L1, L2, L3). - The patient's history of multiple male partners and inconsistent condom use puts him at high risk for acquiring sexually transmitted infections like LGV. *Herpes simplex virus 2* - HSV-2 typically causes **painful, vesicular lesions** that can lead to recurrent outbreaks, which is inconsistent with the initial painless penile lesion described. - While HSV can cause lymphadenopathy, it usually presents as **tender, non-suppurative lymph nodes** rather than large, discharging buboes. *Klebsiella granulomatis* - This organism causes **donovanosis (granuloma inguinale)**, characterized by progressive, painless, beefy red **ulcerative lesions** that bleed easily. - It typically does not involve regional lymphadenopathy or bubo formation, but rather **pseudobuboes** formed by subcutaneous granulomas. *Yersinia pestis* - *Yersinia pestis* is the causative agent of **plague**, a severe systemic illness spread by fleas or respiratory droplets. - While plague can cause **buboes** (swollen, painful lymph nodes), it is associated with a much more acute and severe systemic illness, including high fever, prostration, and often hemorrhagic manifestations, which are not described here. *Haemophilus ducreyi* - This bacterium causes **chancroid**, which presents with **painful, friable genital ulcers** with ragged borders and often associated with tender inguinal lymphadenopathy that can suppurate. - The initial lesion described in the patient was **painless**, which rules out chancroid.

Question 179: An outbreak of diphtheria has occurred for the third time in a decade in a small village in South Africa. Diphtheria is endemic to the area with many healthy villagers colonized with different bacterial strains. Vaccine distribution in this area is difficult due to treacherous terrain. A team of doctors is sent to the region to conduct a health campaign. Toxigenic strains of C. diphtheria are isolated from symptomatic patients. Which of the following best explains the initial emergence of a pathogenic strain causing such outbreaks?

A. Infection with a lytic phage
B. Conjugation between the toxigenic and non-toxigenic strains of C. diphtheriae
C. Suppression of lysogenic cycle
D. Lysogenic conversion (Correct Answer)
E. Presence of naked DNA in the environment

Explanation: ***Lysogenic conversion*** - **Lysogenic conversion** occurs when a temperate bacteriophage infects a bacterium and integrates its DNA into the bacterial genome, carrying genes that confer new properties, such as **toxin production**. - The **diphtheria toxin** gene is encoded by the *tox* gene carried by the **beta-phage**, which integrates into *Corynebacterium diphtheriae* via lysogeny, converting a non-pathogenic strain into a pathogenic one. *Infection with a lytic phage* - A **lytic phage** infects a bacterium, replicates rapidly, and then lyses the host cell, releasing new phage particles; it typically does not integrate into the host genome to confer new stable properties like toxin production. - Lytic phages are primarily responsible for bacterial destruction, not for conferring new stable virulence factors like the **diphtheria toxin**. *Conjugation between the toxigenic and non-toxigenic strains of C. diphtheriae* - **Conjugation** involves the direct transfer of genetic material via a pilus between bacteria, usually involving plasmids. While it can transfer virulence factors, the **diphtheria toxin gene** is chromosomally integrated via a **phage**, not typically transferred through conjugation in this manner. - *C. diphtheriae* toxin production is specifically associated with the presence of the **toxin gene from a lysogenic bacteriophage**, not plasmid-mediated transfer between strains. *Suppression of lysogenic cycle* - **Suppression of the lysogenic cycle** means the phage exits the dormant lysogenic state and enters the lytic cycle, leading to host cell lysis. This would not explain the *initial emergence* or stable acquisition of toxin production. - If the lysogenic cycle were suppressed, the integrated phage (and thus the **toxin gene**) might be lost or the host cell destroyed, rather than stably expressing a new pathogenic trait. *Presence of naked DNA in the environment* - The presence of **naked DNA** in the environment leads to **transformation**, where bacteria take up free DNA from their surroundings. While this can transfer genes, the **diphtheria toxin gene** is specifically introduced into *C. diphtheriae* by a **lysogenic bacteriophage**, not typically by free environmental DNA. - Transformation is a mechanism for acquiring genetic material, but the origin and mechanism of acquisition for the **diphtheria toxin gene** are well-established as phage-mediated.

Question 180: A previously healthy 10-day-old infant is brought to the emergency department by his mother because of episodes of weakness and spasms for the past 12 hours. His mother states that he has also had difficulty feeding and a weak suck. He has not had fever, cough, diarrhea, or vomiting. He was born at 39 weeks' gestation via uncomplicated vaginal delivery at home. Pregnancy was uncomplicated. The mother refused antenatal vaccines out of concern they would cause side effects. She is worried his symptoms may be from some raw honey his older sister maybe inadvertently fed him 5 days ago. He appears irritable. His temperature is 37.1°C (98.8°F). Examination shows generalized muscle stiffness and twitches. His fontanelles are soft and flat. The remainder of the examination shows no abnormalities. Which of the following is the most likely causal organism?

A. Listeria monocytogenes
B. Neisseria meningitidis
C. Escherichia coli
D. Clostridium botulinum (Correct Answer)
E. Clostridium tetani

Explanation: ***Clostridium botulinum*** - The symptoms of **weakness, spasms, difficulty feeding, weak suck**, and history of possible **raw honey ingestion** are highly suggestive of **infant botulism**. - **Infant botulism** occurs when *Clostridium botulinum* spores are ingested and colonize the immature gut, producing **neurotoxins** that cause **descending flaccid paralysis**. - The "muscle stiffness" noted can represent early hypotonia and the **loss of head control** typical of botulism, rather than true spastic rigidity. - **Honey exposure** in infants under 12 months is a classic risk factor due to spore contamination. *Listeria monocytogenes* - This pathogen typically causes **meningitis** or **sepsis** in neonates, with symptoms such as **fever, lethargy**, and **poor feeding**, which differ from the presented neuromuscular symptoms. - While *Listeria* can be transmitted transplacentally or during birth, it would not be directly associated with the ingestion of **honey**. *Neisseria meningitidis* - *N. meningitidis* is a common cause of **bacterial meningitis** and **meningococcemia**, presenting with **fever, rash, irritability**, and **meningeal signs**, which are not the primary symptoms described. - While it can affect infants, it does not typically cause the specific **neuromuscular symptoms** seen in this patient nor is it linked to honey ingestion. *Escherichia coli* - **E. coli** is a frequent cause of **neonatal sepsis** and **meningitis**, often presenting with **fever, poor feeding, lethargy, and vomiting**. - The clinical picture of **weakness, difficulty feeding**, and **neuromuscular symptoms** without significant fever or systemic signs points away from typical *E. coli* infections. *Clostridium tetani* - **Clostridium tetani** causes **tetanus**, characterized by **muscle spasms, rigidity, and lockjaw** (trismus), which represents **spastic paralysis**. - However, the history of **raw honey ingestion** is a classic risk factor for **botulism**, not tetanus. - **Neonatal tetanus** is associated with unhygienic umbilical cord practices, and while **rigidity** is prominent in tetanus, the **flaccid paralysis, weakness**, and **weak suck** are characteristic of **botulism**, not tetanus.

Question 181: A group of medical students is studying bacteria and their pathogenesis. They have identified that a substantial number of bacteria cause human disease by producing exotoxins. Exotoxins are typically proteins, but they have different mechanisms of action and act at different sites. The following is a list of exotoxins together with mechanisms of action. Which of the following pairs is correctly matched?

A. Tetanospasmin - binds 60S ribosome subunit and inhibits protein synthesis
B. Cholera toxin - ADP-ribosylates Gs, keeping adenylate cyclase active and ↑ [cAMP] (Correct Answer)
C. Diphtheria toxin - cleaves synaptobrevin, blocking vesicle formation and the release of acetylcholine
D. Botulinum toxin - cleaves synaptobrevin, blocking vesicle formation and the release of the inhibitory neurotransmitters GABA and glycine
E. Anthrax toxin - ADP-ribosylates elongation factor - 2 (EF-2) and inhibits protein synthesis

Explanation: ***Cholera toxin - ADP-ribosylates Gs, keeping adenylate cyclase active and ↑ [cAMP]*** - **Cholera toxin** works by irrevocably activating **adenylate cyclase** via **ADP-ribosylation** of the **alpha subunit of Gs protein**. - This leads to a persistent increase in intracellular **cyclic AMP (cAMP)**, resulting in excessive secretion of water and electrolytes into the intestinal lumen, causing characteristic **rice-water diarrhea**. *Tetanospasmin - binds 60S ribosome subunit and inhibits protein synthesis* - **Tetanospasmin (tetanus toxin)** acts by cleaving **synaptobrevin**, a SNARE protein, which inhibits the release of **inhibitory neurotransmitters (GABA and glycine)** from Renhaw cells in the spinal cord. - This blockade of inhibitory signals leads to uncontrolled muscle contractions and **spastic paralysis**. *Diphtheria toxin - cleaves synaptobrevin, blocking vesicle formation and the release of acetylcholine* - **Diphtheria toxin** works by **ADP-ribosylating elongation factor-2 (EF-2)**, which is crucial for protein synthesis. - The inactivation of **EF-2** leads to the arrest of protein synthesis and ultimately **cell death**. *Botulinum toxin - cleaves synaptobrevin, blocking vesicle formation and the release of the inhibitory neurotransmitters GABA and glycine* - **Botulinum toxin** cleaves **SNARE proteins** (including synaptobrevin) at the **neuromuscular junction**, specifically blocking the release of **acetylcholine**. - This inhibition of neurotransmitter release at the presynaptic terminal leads to **flaccid paralysis**. *Anthrax toxin - ADP-ribosylates elongation factor - 2 (EF-2) and inhibits protein synthesis* - **Anthrax toxin** consists of three proteins: Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF). The **Edema Factor (EF)** is a **calmodulin-dependent adenylate cyclase** that increases intracellular **cAMP**, and the **Lethal Factor (LF)** is a **metalloprotease** that targets MAPK pathways. - **Anthrax toxin** does not work by ADP-ribosylating EF-2; that mechanism is characteristic of **diphtheria toxin**.

Question 182: A 9-month-old girl is brought in by her father for a scheduled check-up with her pediatrician. He states that over the past 4-5 months she has had multiple ear infections. She was also hospitalized for an upper respiratory infection 2 months ago. Since then she has been well. She has started to pull herself up to walk. Additionally, the patient’s medical history is significant for eczema and allergic rhinitis. The father denies any family history of immunodeficiencies. There are no notable findings on physical exam. Labs are remarkable for low IgG levels with normal IgA, IgE, and IgM levels. Which of the following is the most likely etiology for the patient’s presentation?

A. Failure of B-cell differentiation
B. Defect in Bruton tyrosine kinase
C. Delayed onset of normal immunoglobulins (Correct Answer)
D. Adenosine deaminase deficiency
E. Impaired T cell signaling

Explanation: ***Delayed onset of normal immunoglobulins*** - The patient's presentation with recurrent ear infections, a URI, and low **IgG** levels in the setting of normal **IgA, IgE, and IgM** is characteristic of **transient hypogammaglobulinemia of infancy (THI)**. This resolves spontaneously as the infant's immune system matures. - THI is a condition where the **physiologic nadir** of IgG, typically occurring between 3-6 months, is more prolonged and pronounced than usual, leading to increased susceptibility to infections. *Failure of B-cell differentiation* - This would lead to a more profound impairment of immunoglobulin production, affecting multiple **immunoglobulin classes** (IgA, IgM, IgG) more severely than seen here. - Conditions involving failure of B-cell differentiation, such as **Common Variable Immunodeficiency (CVID)**, typically present later in childhood or adulthood with recurrent severe infections and panhypogammaglobulinemia. *Defect in Bruton tyrosine kinase* - A defect in **Bruton tyrosine kinase (BTK)** is responsible for **X-linked agammaglobulinemia (XLA)**, characterized by an absence or severe deficiency of B cells, and thus **markedly decreased levels of all immunoglobulins** (IgG, IgA, IgM). - The patient here has normal IgA and IgM, and only low IgG, which is not consistent with XLA. *Adenosine deaminase deficiency* - This deficiency causes **severe combined immunodeficiency (SCID)**, affecting both **T-cell and B-cell function**, leading to profoundly low levels of all immunoglobulins and severe, life-threatening infections from early infancy. - The patient's relatively mild course with only recurrent ear infections and a good developmental milestone (pulling to walk) does not fit the SCID picture. *Impaired T cell signaling* - Impaired T cell signaling would result in defects in **cell-mediated immunity** and typically affect **humoral immunity** as well due to the need for T cell help in antibody production. - This would usually lead to low levels of multiple immunoglobulin classes and a broad susceptibility to various pathogens, including **opportunistic infections**, which is not indicated in this case.

Question 183: A 54-year-old woman comes to the physician because of lower back pain, night sweats, and a 5-kg (11-lb) weight loss during the past 4 weeks. She has rheumatoid arthritis treated with adalimumab. Her temperature is 38°C (100.4°F). Physical examination shows tenderness over the T10 and L1 spinous processes. Passive extension of the right hip causes pain in the right lower quadrant. The patient's symptoms are most likely caused by an organism with which of the following virulence factors?

A. Proteins that bind to the Fc region of immunoglobulin G
B. Protease that cleaves immunoglobulin A
C. Polysaccharide capsule that prevents phagocytosis
D. Surface glycolipids that prevent phagolysosome fusion (Correct Answer)
E. Polypeptides that inactivate elongation factor 2

Explanation: ***Surface glycolipids that prevent phagolysosome fusion*** - The patient's symptoms (low back pain, night sweats, weight loss, fever, spinal tenderness, and hip pain) in a patient on **adalimumab** (a TNF-alpha inhibitor) suggest **disseminated tuberculosis** (Pott disease). - *Mycobacterium tuberculosis* uses **mycolic acids** and other surface glycolipids to prevent phagolysosome fusion, allowing it to survive and replicate within macrophages. *Proteins that bind to the Fc region of immunoglobulin G* - This virulence factor is characteristic of bacteria like *Staphylococcus aureus* (Protein A) and *Streptococcus pyogenes* (Protein G), which is not consistent with the clinical picture. - These proteins interfere with opsonization and antibody-mediated immunity, but are not the primary mechanism of *Mycobacterium tuberculosis* survival within macrophages. *Protease that cleaves immunoglobulin A* - **IgA protease** is a virulence factor for bacteria such as *Neisseria gonorrhoeae*, *Neisseria meningitidis*, and *Streptococcus pneumoniae*, which colonize mucosal surfaces. - This mechanism helps these bacteria evade mucosal immunity, but it is not relevant to the pathogenesis of tuberculosis. *Polysaccharide capsule that prevents phagocytosis* - A polysaccharide capsule is a major virulence factor for many encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae*, *Neisseria meningitidis*) that helps them evade phagocytosis. - However, *Mycobacterium tuberculosis* is not primarily characterized by a polysaccharide capsule for immune evasion; its internal survival within macrophages is more critical. *Polypeptides that inactivate elongation factor 2* - Toxins that inactivate **elongation factor 2** are associated with *Corynebacterium diphtheriae* (**diphtheria toxin**) and *Pseudomonas aeruginosa* (**exotoxin A**), leading to inhibition of protein synthesis. - This mechanism is not involved in the pathogenesis of *Mycobacterium tuberculosis* infection or its ability to cause disseminated disease.

Question 184: A 3-year-old recent immigrant is diagnosed with primary tuberculosis. Her body produces T cells that do not have IL-12 receptors on their surface, and she is noted to have impaired development of Th1 T-helper cells. Which of the following cytokines would benefit this patient?

A. IL-4
B. IL-22
C. TGF-beta
D. IL-17
E. Interferon-gamma (Correct Answer)

Explanation: ***Interferon-gamma*** - This patient has an impaired **Th1 response**, which is crucial for controlling intracellular infections like **tuberculosis** by activating macrophages. - **Interferon-gamma** is the primary cytokine produced by **Th1 cells** that activates macrophages, leading to enhanced phagocytosis and killing of intracellular pathogens. *IL-4* - **IL-4** is a key cytokine produced by **Th2 cells**, which primarily drive **humoral immunity** and allergic responses, not cellular immunity against intracellular bacteria. - Its administration would promote a **Th2 response**, which is not beneficial for combating **tuberculosis** and might even suppress the desirable Th1 response. *IL-22* - **IL-22** is mainly involved in **epithelial cell proliferation**, host defense at mucosal surfaces, and tissue repair. - While it has a role in host defense against certain pathogens, it is not the primary cytokine required to compensate for a deficient **Th1 response** in **tuberculosis**. *TGF-beta* - **TGF-beta** is a pleiotropic cytokine with roles in cell growth, differentiation, and immune regulation, often acting as an **immunosuppressant** or driving **Treg differentiation**. - It would not directly compensate for a lack of **Th1 cell function** needed to activate macrophages against **tuberculosis**. *IL-17* - **IL-17** is the signature cytokine of **Th17 cells**, which are important for host defense against **extracellular bacteria** and fungi, primarily by recruiting neutrophils. - While it plays a role in some immune responses, it is not the crucial cytokine to boost in a patient with impaired **Th1 development** against **intracellular M. tuberculosis**.

Question 185: A 35-year-old patient with no significant past medical history arrives to the ED with abdominal cramps, nausea, and vomiting. He has had no recent travel or chemical exposures; however, three other members of his family also arrived concurrently to the ED with abdominal cramps, nausea, and vomiting. When asked about their recent activities, they recall that they had shared a lunch of leftover fried rice and soft boiled eggs about 5 hours earlier. The patients are otherwise afebrile and deny any history of diarrhea. Which of the following toxins is the most likely to have caused these symptoms?

A. Shiga toxin
B. Cereulide (Correct Answer)
C. Endotoxin
D. Exotoxin A
E. Toxin B

Explanation: ***Cereulide*** - The rapid onset (within 5 hours) of gastrointestinal symptoms (nausea, vomiting, abdominal cramps) after consuming **fried rice** and **boiled eggs** is characteristic of intoxication by **cereulide**. - This preformed toxin is produced by *Bacillus cereus* in improperly stored starchy foods like rice, leading to emetic-type food poisoning. *Shiga toxin* - **Shiga toxin** is produced by *Shigella dysenteriae* and Shiga toxin-producing *E. coli* (STEC), typically causing **bloody diarrhea** and **hemolytic uremic syndrome (HUS)**, which are not present here. - The incubation period for Shiga toxin-mediated illness is usually longer, ranging from 1 to 8 days, making a rapid onset of symptoms unlikely. *Endotoxin* - **Endotoxin** (lipopolysaccharide or LPS) is a component of the outer membrane of **Gram-negative bacteria** and causes systemic symptoms like fever, shock, and organ dysfunction when released during bacterial lysis. - While it can cause some gastrointestinal effects, the rapid onset of isolated emetic symptoms in a food poisoning cluster is not typical for endotoxin as the primary cause. *Exotoxin A* - **Exotoxin A** is a virulence factor produced by *Pseudomonas aeruginosa* and is associated with severe infections in immunocompromised patients, not typically foodborne illness. - Its mechanism of action involves inhibiting protein synthesis, leading to tissue damage in specific infections, not acute emetic food poisoning. *Toxin B* - **Toxin B** is produced by *Clostridioides difficile* and is a major cause of **antibiotic-associated colitis** and **pseudomembranous colitis**, characterized by diarrhea and abdominal pain. - The patient's presentation of acute onset nausea, vomiting, and abdominal cramps without diarrhea and no history of antibiotic use does not align with *C. difficile* infection.

Question 186: A 4-year-old male is brought by his mother to the emergency room with dyspnea and fever. His mother reports a two-day history of progressive shortness of breath, malaise, and a fever with a maximum temperature of 101.6°F (38.7°C). The child has visited the emergency room three times over the past two years for pneumonia and otitis media. His family history is notable for sarcoidosis in his mother, diabetes in his father, and an early childhood death in his maternal uncle. His temperature is 101.2°F (38.4°C), blood pressure is 110/90 mmHg, pulse is 110/min, and respirations are 24/min. Physical examination reveals scant lymphoid tissue. A serological analysis reveals decreased levels of IgA, IgG, and IgM. This patient most likely has a defect in a protein that is active in which of the following cellular stages?

A. Mature B-cell
B. Pro-B-cell
C. Immature B-cell
D. Pre-B-cell (Correct Answer)
E. Plasma cell

Explanation: ***Pre-B-cell*** - The patient's history of recurrent infections (pneumonia, otitis media), scant lymphoid tissue, and pan-hypogammaglobulinemia (decreased IgA, IgG, IgM) are classic findings of **X-linked agammaglobulinemia (XLA)**. - XLA is caused by a defect in **Bruton's tyrosine kinase (BTK)**, which is crucial for B-cell maturation at the **pre-B-cell stage**, preventing them from developing into mature B lymphocytes and subsequently plasma cells. *Mature B-cell* - A defect at the mature B-cell stage would likely present with different patterns of immunoglobulin deficiencies, possibly affecting specific Ig classes or leading to lymphoproliferative disorders. - In XLA, the arrest occurs earlier, preventing the formation of mature B cells altogether, hence the profound lack of all immunoglobulin classes. *Pro-B-cell* - Defects at the **pro-B-cell stage** would lead to an even more severe block in B-cell development, potentially preventing the formation of pre-B cells. - While significant, the specific clinical picture and known genetic defects point more distinctly to an issue at or just before the pre-B cell stage. *Immature B-cell* - A block at the immature B-cell stage would allow for the presence of pre-B cells, but issues in developing tolerance or further maturation. - The defining feature of XLA is the absence of mature B cells and immunoglobulins, which results from the failure of pre-B cells to develop into immature B cells. *Plasma cell* - Plasma cells are the final differentiated form of B cells responsible for antibody production. - A defect solely at the plasma cell stage would imply normal B-cell development up to that point, which is inconsistent with the absence of all immunoglobulin classes and scant lymphoid tissue seen in XLA.

Question 187: An 8-month-old boy is brought to his pediatrician by his parents with a 12-hour history of fever and coughing. He has also been experiencing intermittent diarrhea and skin abscesses since birth. Otherwise, he has been meeting developmental milestones as expected. Analysis of this patient's sputum reveals acute angle branching fungi, and culture shows gram-positive cocci in clusters. A flow cytometry reduction test was obtained that confirmed the diagnosis. Which of the following processes is most likely defective in this patient?

A. Actin polymerization
B. Transforming oxygen into superoxide radicals (Correct Answer)
C. Transforming superoxide radicals into hydrogen peroxide
D. Leukocyte migration
E. Maturation of B-cells

Explanation: ***Transforming oxygen into superoxide radicals*** - The patient's history of **recurrent fungal and bacterial infections**, specifically with **Staphylococcus aureus** (gram-positive cocci in clusters leading to abscesses) and **Aspergillus** (acute angle branching fungi), is highly suggestive of **Chronic Granulomatous Disease (CGD)**. - CGD is characterized by a defect in **NADPH oxidase**, which is responsible for converting **oxygen into superoxide radicals** during the **respiratory burst** in phagocytes, a critical step for killing ingested microbes. The flow cytometry reduction test (e.g., DHR test) confirms this diagnosis by showing reduced or absent superoxide production. *Actin polymerization* - Defects in actin polymerization are associated with disorders affecting **cytoskeletal function**, such as **Wiskott-Aldrich syndrome**, which presents with **thrombocytopenia**, **eczema**, and **recurrent infections**. - While infections are part of the presentation, the specific pattern of **abscesses** and **fungal infections** with a positive flow cytometry reduction test for phagocyte function points away from a primary defect in actin dynamics. *Transforming superoxide radicals into hydrogen peroxide* - This process is catalyzed by **superoxide dismutase (SOD)**, which converts superoxide radicals into hydrogen peroxide and oxygen. - In CGD, the primary defect is the **failure to generate superoxide radicals in the first place** due to NADPH oxidase deficiency, not a problem with their subsequent conversion. The flow cytometry reduction test specifically demonstrates the inability to produce superoxide, confirming this upstream defect. *Leukocyte migration* - **Leukocyte adhesion deficiencies (LADs)** involve impaired leukocyte migration due to defects in adhesion molecules like **integrins**. - This would present with **delayed umbilical cord separation**, **recurrent bacterial infections without pus formation**, and **leukocytosis**, which are not the primary features described here. *Maturation of B-cells* - Defects in B-cell maturation lead to various forms of **primary immunodeficiency** characterized by **antibody deficiencies**, such as **X-linked agammaglobulinemia**. - These conditions typically present with **recurrent bacterial infections** (especially encapsulated bacteria) but generally do not involve the characteristic **granulomatous inflammation** or vulnerability to specific opportunistic pathogens seen in CGD.

Question 188: A 32 year-old African American man with a past medical history of sickle cell anemia presents to his primary care physician for a routine check-up. He has no complaints. His physician notes that he likely has an increased susceptibility to infections with certain organisms. Which of the following patient groups has a similar pattern of increased susceptibility?

A. Hemophilia B
B. C5-9 complement deficiency
C. T-cell deficiency
D. Hemophilia A
E. C3 complement deficiency (Correct Answer)

Explanation: ***C3 complement deficiency*** - Patients with **sickle cell anemia** often experience **autosplenectomy** due to recurrent vaso-occlusive crises, leading to functional asplenia and impaired complement-mediated opsonization and clearance of encapsulated bacteria. A **C3 complement deficiency** directly impairs the entire complement cascade (both classical and alternative pathways), resulting in a similar defect in **opsonization** and clearance, particularly for **encapsulated bacteria**. - Both conditions lead to an increased susceptibility to infections with encapsulated bacteria such as *Streptococcus pneumoniae*, *Haemophilus influenzae type b*, and *Neisseria meningitidis* due to the impaired ability to clear these pathogens from the bloodstream. *Hemophilia B* - This is an **X-linked recessive bleeding disorder** caused by a deficiency in coagulation factor IX. - It results in **impaired clotting** and an increased risk of bleeding, not an increased susceptibility to specific infections. *C5-9 complement deficiency* - A deficiency in **C5-9 complement components** specifically impairs the formation of the **membrane attack complex (MAC)**. - This primarily leads to an increased susceptibility to infections with **Neisseria species** (e.g., *N. meningitidis*, *N. gonorrhoeae*), as MAC is crucial for lysing these gram-negative bacteria. While C3 deficiency also increases susceptibility to *Neisseria*, C5-9 deficiency is more specifically and severely linked to this pathogen. *T-cell deficiency* - **T-cell deficiencies** (e.g., DiGeorge syndrome, HIV/AIDS) lead to impaired **cell-mediated immunity**. - This results in increased susceptibility to **intracellular pathogens**, **viruses**, **fungi**, and **opportunistic infections** (e.g., *Pneumocystis jirovecii*, *Candida*, CMV), which is distinct from the encapsulated bacterial infections seen in sickle cell anemia. *Hemophilia A* - This is an **X-linked recessive bleeding disorder** caused by a deficiency in coagulation factor VIII. - Similar to Hemophilia B, it primarily causes **bleeding abnormalities** due to dysfunctional secondary hemostasis, not a heightened susceptibility to infections.

Question 189: A 7-year-old boy presents with difficult left eye-opening in the morning, eye discharge, and irritation. These symptoms developed gradually over the past week. He attends a primary school where recently an outbreak of tonsillitis took place. He had otitis media 2 weeks ago treated with ampicillin. At the presentation, the patient’s vital signs are within normal limits. Eye examination reveals bulbar conjunctival injection, mild eyelid edema, and a moderate mucopurulent discharge with crusts on the lower eyelid. There is no corneal or eyelid ulceration. No lymphadenopathy is noted. Which of the following investigations should be performed to establish a diagnosis before the treatment?

A. Scrapings with Gram staining
B. Rapid viral test
C. Polymerase chain reaction
D. Bacterial culture of the discharge
E. No investigations are required in this case (Correct Answer)

Explanation: ***No investigations are required in this case*** - This presentation is highly suggestive of **bacterial conjunctivitis**, characterized by **mucopurulent discharge**, **eyelid edema**, and **crusting**, especially in a child. The history of a recent tonsillitis outbreak and otitis media, while not directly causative for the conjunctivitis, indicates an environment where bacterial infections are circulating. - In cases of typical acute bacterial conjunctivitis, especially in children, treatment (usually topical antibiotics) is often initiated empirically without prior microbiologic investigation, as the diagnosis is clear clinically and waiting for results can delay appropriate care. *Scrapings with Gram staining* - While Gram staining can identify bacteria, it is generally **not necessary for routine acute bacterial conjunctivitis** given the typical clinical picture. Empirical treatment is usually sufficient. - It might be considered in cases of **severe, recurrent, or atypical conjunctivitis**, or if initial treatment fails, to guide antibiotic selection. *Rapid viral test* - The symptoms, particularly the **mucopurulent discharge** and **crusting**, are more indicative of a bacterial rather than a viral infection. Viral conjunctivitis typically presents with a watery discharge. - A rapid viral test would likely be **negative** and is not indicated given the clinical presentation predominantly suggesting bacterial etiology. *Polymerase chain reaction* - PCR is a highly sensitive test for identifying pathogens but is **overkill for a straightforward case of bacterial conjunctivitis**. It's generally reserved for specific, hard-to-diagnose infections or research. - Its use would **delay treatment** and is not cost-effective for a condition that can be managed empirically. *Bacterial culture of the discharge* - Bacterial culture, like Gram staining, is typically **not required for initial management** of acute bacterial conjunctivitis, as the condition often resolves with empirical antibiotic treatment. - It is usually reserved for cases that are **severe, chronic, non-responsive to initial therapy, or recurrent**, to determine antibiotic sensitivity.

Question 190: A family doctor in a rural area is treating a patient for dyspepsia. The patient had chronic heartburn and abdominal pain for the last 2 months and peptic ulcer disease due to a suspected H. pylori infection. For reasons relating to affordability and accessibility, the doctor decides to perform a diagnostic test in the office that is less invasive and more convenient. Which of the following is the most likely test used?

A. Steiner's stain
B. Culture of organisms from gastric specimen
C. Stool antigen test (Correct Answer)
D. Detection of the breakdown products of urea in biopsy
E. Serology (ELISA testing)

Explanation: ***Stool antigen test*** - This **non-invasive** and **cost-effective** test detects *H. pylori* antigens in stool, making it suitable for a rural setting with limited resources. - It is highly sensitive and specific, useful for both initial diagnosis and confirming eradication after treatment. *Steiner's stain* - **Steiner's stain** (Steiner silver stain) is primarily used for histological visualization of *Legionella* species, and **not for** *H. pylori* detection in routine clinical practice. - It requires an **endoscopic biopsy**, making it more invasive and costly than the stool antigen test. *Culture of organisms from gastric specimen* - This method requires an **endoscopic biopsy** and specialized culture facilities, which may not be available in a rural doctor's office. - It is more expensive and time-consuming, and primarily used when **antibiotic resistance** is suspected. *Detection of the breakdown products of urea in biopsy* - This refers to the **rapid urease test** (e.g., CLOtest), which is performed on a **gastric biopsy** obtained during endoscopy. - While quick, it is an **invasive procedure** requiring endoscopy, which contradicts the patient's and doctor's preferences for a less invasive test. *Serology (ELISA testing)* - **Serology** detects antibodies to *H. pylori* but cannot differentiate between **active infection** and **past exposure**. - Its utility in monitoring eradication is limited, and it's generally not recommended as the primary diagnostic test due to its inability to confirm active infection.

Question 191: To protect against a potentially deadly infection, a 19-year-old female receives a vaccine containing capsular polysaccharide. This vaccine will stimulate her immune system to produce antibodies against which organism?

A. Corynebacterium diphtheriae
B. Neisseria meningitidis (Correct Answer)
C. Streptococcus pneumoniae
D. Clostridium tetani
E. Haemophilus influenzae type b

Explanation: ***Neisseria meningitidis*** - The vaccine described, containing **capsular polysaccharide**, targets the **polysaccharide capsule** of *Neisseria meningitidis*, which is a key virulence factor. - This bacterium causes **meningococcal meningitis**, a potentially deadly infection, especially in adolescents and young adults. - The **meningococcal vaccine** is specifically recommended for adolescents and college students due to increased risk in this population. *Corynebacterium diphtheriae* - The vaccine against *C. diphtheriae* is a **toxoid vaccine**, meaning it contains an inactivated form of the **diphtheria toxin**, not capsular polysaccharide. - This vaccine primarily protects against the effects of the **exotoxin**, which causes major symptoms like myocarditis and neuropathy. *Streptococcus pneumoniae* - While *S. pneumoniae* also has a **capsular polysaccharide vaccine** (PPSV23 and PCV13), it is primarily recommended for **young children, elderly adults, and immunocompromised patients**. - A **19-year-old healthy female** would not routinely receive pneumococcal vaccine unless she had specific risk factors. - The question context of a "potentially deadly infection" in this age group more specifically points to meningococcus. *Clostridium tetani* - The vaccine for *C. tetani* is a **tetanus toxoid vaccine**, similar to diphtheria, targeting the inactivated **tetanospasmin toxin** produced by the bacterium. - This vaccine prevents the neurological symptoms of tetanus by neutralizing the toxin, not by targeting capsular polysaccharides. *Haemophilus influenzae type b* - While *H. influenzae* type b also has a **capsular polysaccharide-based vaccine** (conjugate vaccine), it is primarily given to **infants and young children** as part of routine childhood immunization. - A 19-year-old would have already received this vaccine in childhood, and it is not routinely given to adolescents or adults. - The age group and clinical context make meningococcus the more likely answer.

Question 192: A 42-year-old woman presents to a medical office with complaints of fatigue, weight loss, and low-grade fever for 1 week. She noticed bleeding spots on her feet this morning. The past medical history is significant for a recent dental appointment. She is a non-smoker and does not drink alcohol. She does not currently take any medications. On examination, the vital signs include temperature 37.8°C (100.0°F), blood pressure 138/90 mm Hg, respirations 21/min, and pulse 87/min. Cardiac auscultation reveals a pansystolic murmur in the mitral area with radiation to the left axilla. Laboratory studies show hemoglobin levels of 17.2 g/dL, erythrocyte sedimentation rate (ESR) of 25 mm/h, and a white blood cell (WBC) count of 12,000 cells/mm3. An echocardiogram reveals valvular vegetations on the mitral valve with mild regurgitation. Blood samples are sent for bacterial culture. Empiric antibiotic therapy is initiated with ceftriaxone and vancomycin. The blood cultures most likely will yield the growth of which of the following organisms?

A. Actinomyces israelii
B. Coxiella burnetii
C. Group B Streptococcus
D. Staphylococcus aureus
E. Streptococcus viridans (Correct Answer)

Explanation: ***Streptococcus viridans*** - The patient's recent **dental appointment**, the presence of an **oral organism**, and symptoms consistent with **subacute bacterial endocarditis** (fatigue, fever, weight loss, petechiae) strongly suggest *S. viridans* as the causative agent. - *Streptococcus viridans* species are common inhabitants of the **oral flora** and are a leading cause of endocarditis following dental procedures, especially in individuals with pre-existing valvular abnormalities. *Actinomyces israelii* - While *Actinomyces israelii* is an oral commensal, it typically causes **actinomycosis**, a chronic, suppurative infection characterized by abscess formation and sinus tracts, often following dental procedures. - It less commonly presents as acute or subacute endocarditis and would not typically cause the rapid progression of symptoms described. *Coxiella burnetii* - *Coxiella burnetii* causes **Q fever**, which can manifest as endocarditis, often associated with exposure to **farm animals** or their products. - The patient's history lacks any such exposure, and the clinical presentation is more aligned with standard bacterial endocarditis from oral flora. *Group B Streptococcus* - **Group B Streptococcus** (*Streptococcus agalactiae*) is primarily known as a cause of **neonatal sepsis** and meningitis, and infections in immunocompromised adults or those with underlying conditions like diabetes. - It is not typically associated with endocarditis following a dental procedure in an otherwise healthy adult. *Staphylococcus aureus* - **Staphylococcus aureus** can cause endocarditis, particularly in **intravenous drug users** or patients with prosthetic valves/indwelling lines, and often presents as a more **acute and aggressive disease**. - While possible, the association with a recent dental procedure and the subacute course makes *Streptococcus viridans* a more likely culprit in this specific scenario.

Question 193: A 47-year-old alcoholic man presents to the office for a 72-hour history of intense right hemithorax pain. He also complains of fever, chills, and sweating that started 10 days ago, as well as persistent coughing with abundant malodorous sputum. Chest radiography shows a round consolidation with air-fluid levels in the middle third of the right hemithorax. Sputum samples for a direct exam, culture, and acid-fast bacilli (AFB) are sent. What is the correct diagnosis?

A. Bronchopulmonary sequestration
B. Bronchiectasis
C. Tuberculosis
D. Lung cancer
E. Lung abscess (Correct Answer)

Explanation: ***Lung abscess*** - The combination of **fever, chills, sweating, persistent coughing with malodorous sputum**, and a **round consolidation with air-fluid levels** on chest radiography in an alcoholic patient is highly indicative of a lung abscess. - Alcoholism is a significant risk factor due to aspiration, leading to polymicrobial infections and subsequent tissue necrosis and cavitation. *Bronchopulmonary sequestration* - This is a rare congenital malformation where a portion of lung tissue is not connected to the tracheobronchial tree and receives its blood supply from the systemic circulation. - It typically presents as a **mass lesion** on imaging, often without acute signs of infection like fever and malodorous sputum, unless it becomes secondarily infected. *Bronchiectasis* - Characterized by **irreversible dilation of the bronchi** resulting from chronic inflammation and infection, leading to chronic cough and sputum production. - While it can cause chronic cough and sputum, the acute presentation with a distinct **round consolidation with air-fluid levels** points more specifically to an abscess. *Tuberculosis* - While tuberculosis can cause **fever, cough, and cavitary lesions** which might appear as air-fluid levels, the **malodorous sputum** is less typical, and the acute onset (72-hour intense pain after 10 days of symptoms) is more consistent with a pyogenic bacterial infection. - Furthermore, a round consolidation with air-fluid levels without other classic features of TB (e.g., upper lobe predominance, Ghon complex) makes it less likely given the other findings. *Lung cancer* - Lung cancer can present as a **mass or consolidation** on imaging, and patients may experience cough, weight loss, and fatigue. - However, the rapid onset of **fever, chills, malodorous sputum, and air-fluid levels** strongly suggests an infectious process with cavitation rather than an uncomplicated malignancy.

Question 194: During a humanitarian medical mission in rural Vietnam, a medical resident encounters a 50-year-old man with a year-long history of a pruritic rash on his upper body and face, along with numbness and tingling sensation of both of his palms. He mostly works on his family’s rice farm, where he also takes care of livestock. A physical examination revealed multiple erythematous macules and papules on the face, arms, chest, and back, as well as thinning of the eyebrows and loss of some eyelashes. Additional findings include hypopigmented macules around the elbows, which are insensitive to light touch, temperature, and pinprick. The grip strength is slightly diminished bilaterally with the conservation of both bicipital reflexes. What is the most likely diagnosis?

A. Leprosy (Correct Answer)
B. Sporotrichosis
C. Tinea corporis
D. Scrofula
E. Cutaneous leishmaniasis

Explanation: ***Leprosy*** - The combination of a **chronic pruritic rash**, **sensory loss** (numbness, tingling, insensitivity to touch/temperature/pinprick) in hypopigmented macules, **thinning eyebrows**, and **loss of eyelashes (madarosis)** points strongly to leprosy. - The patient's long-term exposure in a rural, livestock-intensive environment in Vietnam is consistent with areas where **leprosy (Hansen's disease)** is endemic. - **Peripheral neuropathy** with motor involvement (diminished grip strength) and preserved reflexes is characteristic. *Sporotrichosis* - Typically presents as **subcutaneous nodules** or ulcers, often in a **lymphocutaneous pattern**, following a puncture wound. - While it can occur in agricultural workers, it doesn't usually cause widespread pruritic macules/papules, nerve involvement, or loss of eyebrows/eyelashes. *Tinea corporis* - Characterized by **annular, scaly, erythematous patches** with central clearing, often itchy. - It does not cause sensory deficits, thinning of eyebrows/eyelashes, or hypopigmented, anesthetic lesions. *Scrofula* - This refers to **tuberculosis lymphadenitis**, primarily affecting the cervical lymph nodes, causing chronic swelling and sometimes draining sinuses. - It does not present with a widespread pruritic rash, sensory neuropathy, or characteristic skin lesions like those described in the patient. *Cutaneous leishmaniasis* - Causes persistent **skin lesions (papules, nodules, ulcers)**, often with a raised border, typically following a bite from a sandfly. - While it can be chronic and occur in endemic areas, it generally does not cause sensory nerve damage, eyebrow/eyelash loss, or widespread hypopigmented anesthetic macules.

Question 195: A previously healthy 5-year-old boy is brought to the physician with a recurring fever and malaise for 3 weeks. He has also had fatigue and loss of appetite. He initially presented 2 weeks ago with a maculopapular rash that has since resolved. At the time, he was given a prescription for amoxicillin-clavulanate. He denies sore throat or myalgias. He is home-schooled and has had no sick contacts. There are no pets at home, but he often visits a feline animal shelter where his mother volunteers. His temperature is 38.4°C (101.2°F). Physical examination shows a 1-cm papular lesion on the back of the right hand. He also has tender, bulky lymphadenopathy of the axillae and groin. Which of the following is the most appropriate next step in management?

A. Azithromycin therapy (Correct Answer)
B. Streptomycin therapy
C. Doxycycline therapy
D. Itraconazole therapy
E. Pyrimethamine therapy

Explanation: ***Azithromycin therapy*** - This patient's symptoms (fever, malaise, fatigue, papular lesion, generalized lymphadenopathy) combined with exposure to cats strongly suggest **cat-scratch disease (CSD)**, caused by *Bartonella henselae*. - **Azithromycin** is the preferred treatment for CSD, especially in children, due to its efficacy and good tolerability. *Streptomycin therapy* - **Streptomycin** is an aminoglycoside primarily used for severe bacterial infections, particularly mycobacterial infections and some Gram-negative bacteria, but it is **not indicated for *Bartonella henselae***. - It has significant side effects, including **ototoxicity** and **nephrotoxicity**, making it an inappropriate first-line choice for CSD. *Doxycycline therapy* - While **doxycycline** is effective against *Bartonella henselae* and can be used in CSD, it is generally **avoided in children under 8 years old** due to the risk of permanent **tooth discoloration** and inhibition of bone growth. - Therefore, azithromycin is a more suitable choice for a 5-year-old. *Itraconazole therapy* - **Itraconazole** is an **antifungal medication** used to treat systemic fungal infections, such as histoplasmosis, blastomycosis, and aspergillosis. - It has **no activity against bacteria** like *Bartonella henselae*, making it ineffective for cat-scratch disease. *Pyrimethamine therapy* - **Pyrimethamine** is an **antiparasitic medication** typically used in combination with sulfadiazine for conditions like **toxoplasmosis** and some forms of malaria. - It is **not effective against bacterial infections**, including those caused by *Bartonella henselae*.

Question 196: A 28-year-old man presents with a draining abscess on his left jaw. The patient states that he had a “bad tooth” a few weeks ago which has progressed to his current condition. His vital signs include: blood pressure 110/80 mm Hg, heart rate 85/min, and temperature 37.9°C (100.3°F). On physical examination, the patient has a 4 cm abscess on the left maxillary line that is draining a granulous, purulent material. Which of the following is the most likely causative organism of this abscess?

A. Gram-positive cocci in clusters
B. Gram-positive cocci in chains
C. Enveloped, double stranded DNA virus
D. Gram-positive, branching rod (Correct Answer)
E. Aerobic gram-negative rod

Explanation: ***Gram-positive, branching rod*** - The description of a **draining abscess** on the jaw originating from a "bad tooth," with **granulous, purulent material**, is highly suggestive of **actinomycosis**. - *Actinomyces israelii*, the most common causative agent, is a **Gram-positive, branching rod** that forms characteristic **sulfur granules** in pus, which are macroscopic colonies of bacteria and calcium phosphate. *Gram-positive cocci in clusters* - This description typically refers to **Staphylococcus aureus**, which causes abscesses, but they are usually **acute, painful, and rapidly developing**, without the characteristic "sulfur granules" seen in actinomycosis. - While *S. aureus* can certainly infect the oral cavity, the **chronic nature** and **"granulous" discharge** point away from a typical staphylococcal infection. *Gram-positive cocci in chains* - This typically describes **Streptococcus species**, which are common oral flora and can cause dental infections and cellulitis. - However, they do not typically form the **granulous, draining abscesses** or **sulfur granules** associated with actinomycosis. *Enveloped, double stranded DNA virus* - This category includes viruses such as herpesviruses or poxviruses; viruses do not cause **bacterial abscesses** with purulent, granulous material. - Viral infections present with different clinical manifestations, such as **vesicles, ulcers**, or systemic disease, not a chronic draining abscess. *Aerobic gram-negative rod* - While some **aerobic Gram-negative rods** (e.g., *Pseudomonas*, *E. coli*) can cause abscesses, especially in immunocompromised individuals or hospital settings, they are less common causes of **odontogenic abscesses** in healthy patients. - These bacteria do not produce the **sulfur granules** characteristic of actinomycosis.

Question 197: A 21-year-old U.S. born first year medical student with no prior hospital or healthcare work presents to the physician for a routine physical exam. The patient is HIV negative, denies drug use, and denies sick contacts. The physician places a purified protein tuberculin test in the patient's right forearm intradermally. What is the proper time to read the test and induration diameter that would indicate a positive test result?

A. 36 hours and 7mm diameter
B. 48 hours and 11mm diameter
C. 72 hours and 16mm diameter (Correct Answer)
D. 96 hours and 14mm diameter
E. 24 hours and 18mm diameter

Explanation: ***72 hours and 16mm diameter*** - The **purified protein derivative (PPD) test** should ideally be read between 48 and 72 hours after administration to allow for the **Type IV hypersensitivity reaction** to fully develop. - For individuals with no known risk factors for tuberculosis and no prior exposure, an induration of **≥15 mm** is considered a positive result. A 16mm diameter falls within this range. *36 hours and 7mm diameter* - **36 hours** is too early to accurately read a PPD test, as the delayed-type hypersensitivity reaction may not have fully manifested. - A **7mm induration** would generally be considered negative in a low-risk individual, as the threshold for positivity in this group is higher. *48 hours and 11mm diameter* - While **48 hours** is within the acceptable window for reading a PPD test, an **11mm induration** is not considered positive for a young, low-risk individual without any predisposing conditions like HIV or organ transplant. - The threshold for a positive result in this demographic is typically **≥15 mm**. *96 hours and 14mm diameter* - **96 hours** (4 days) is generally too late to accurately read a PPD test, as the reaction may begin to fade, leading to a potentially false negative. - A **14mm induration** is still below the positive threshold of ≥15mm for a low-risk individual. *24 hours and 18mm diameter* - **24 hours** is significantly too early to read a PPD test, as the immune response will not have fully developed, leading to unreliable results. - While **18mm induration** would be a positive result, the timing makes the reading invalid.

Question 198: A 43-year-old Caucasian male spent the past month on a business trip in the Caribbean. Two weeks following his return, he began experiencing diarrhea, pain in his abdomen, and a headache. He presents to the hospital and is noted to be febrile with prominent rose-colored spots on his chest and abdomen. Following recovery, the patient may become a carrier of the bacteria with the bacteria heavily localized to the:

A. Lungs
B. CD4 T-helper cells
C. Gallbladder (Correct Answer)
D. Sensory ganglia
E. Spleen

Explanation: ***Gallbladder*** - The clinical picture (travel to Caribbean, **diarrhea, abdominal pain, fever, rose spots**) strongly suggests **typhoid fever** caused by *Salmonella typhi*. - Individuals who recover from typhoid fever can become **chronic carriers**, with the bacteria persisting primarily in the **gallbladder**, leading to intermittent shedding in feces. *Lungs* - While respiratory symptoms can occur in severe cases, the **lungs** are not the primary site of chronic carriage for *Salmonella typhi*. - **Pneumonia** or other respiratory infections can be complications but not the reservoir for long-term asymptomatic carriage. *CD4 T-helper cells* - **CD4 T-helper cells** are primarily targeted by viruses like **HIV**, not bacterial infections like *Salmonella typhi*. - *Salmonella typhi* is an **intracellular pathogen** but typically infects macrophages and other phagocytic cells, not CD4 T-helper cells for chronic carriage. *Sensory ganglia* - **Sensory ganglia** are associated with latent viral infections like **herpes zoster** (chickenpox/shingles) and are not a site for *Salmonella typhi* persistence. - Bacterial carriage in ganglia is not a known mechanism for typhoid fever. *Spleen* - The **spleen** can be affected during the acute phase of typhoid fever (e.g., splenomegaly) and can harbor *Salmonella typhi* acutely. - However, while involved during infection, it is not the classic or most significant long-term reservoir for chronic carriage, which is primarily the gallbladder.

Question 199: Three days after undergoing coronary bypass surgery, a 67-year-old man becomes unresponsive and hypotensive. He is intubated, mechanically ventilated, and a central line is inserted. Vasopressin and noradrenaline infusions are begun. A Foley catheter is placed. Six days later, he has high unrelenting fevers. He is currently receiving noradrenaline via an infusion pump. His temperature is 39.6°C (102.3°F), pulse is 113/min, and blood pressure is 90/50 mm Hg. Examination shows a sternal wound with surrounding erythema; there is no discharge from the wound. Crackles are heard at both lung bases. Cardiac examination shows an S3 gallop. Abdominal examination shows no abnormalities. A Foley catheter is present. His hemoglobin concentration is 10.8 g/dL, leukocyte count is 21,700/mm3, and platelet count is 165,000/mm3. Samples for blood culture are drawn simultaneously from the central line and peripheral IV line. Blood cultures from the central line show coagulase-negative cocci in clusters on the 8th postoperative day, and those from the peripheral venous line show coagulase-negative cocci in clusters on the 10th postoperative day. Which of the following is the most likely diagnosis in this patient?

A. Central line-associated blood stream infection (Correct Answer)
B. Catheter-associated urinary tract infection
C. Surgical site infection
D. Bowel ischemia
E. Ventilator-associated pneumonia

Explanation: ***Central line-associated blood stream infection*** - The patient exhibits signs of **sepsis** (fever, hypotension, tachycardia) following central line insertion, and **coagulase-negative cocci** (e.g., *Staphylococcus epidermidis*) were isolated from both central and peripheral blood cultures with differential times to positivity, indicating a central line origin. - The organism isolated, **coagulase-negative cocci**, is a common cause of **central line-associated bloodstream infections** (CLABSI). *Catheter-associated urinary tract infection* - While a **Foley catheter** is present, there are no specific signs or symptoms of a **urinary tract infection**, such as dysuria, frequency, or hematuria. - The microbiology results (coagulase-negative cocci in blood, not urine) do not support a urinary source for the infection. *Surgical site infection* - There is **erythema** around the sternal wound, suggesting a superficial infection, but no **discharge** or deepening wound involvement is noted. - A surgical site infection would typically manifest with more prominent localized signs and would be less likely to cause a systemic bloodstream infection with coagulase-negative cocci detected *before* peripheral line cultures. *Bowel ischemia* - This condition is often associated with **abdominal pain**, distension, and signs of organ dysfunction. - The abdominal examination is explicitly stated as normal, making bowel ischemia unlikely. *Ventilator-associated pneumonia* - The patient has crackles at lung bases and is mechanically ventilated, but there are no specific findings like new infiltrates on chest imaging or purulent sputum that would strongly indicate **pneumonia**. - The isolated organism in the blood (coagulase-negative cocci) is not a typical pathogen for ventilator-associated pneumonia, which usually involves Gram-negative rods or *Staphylococcus aureus*.

Question 200: A group of microbiological investigators is studying bacterial DNA replication in E. coli colonies. While the cells are actively proliferating, the investigators stop the bacterial cell cycle during S phase and isolate an enzyme involved in DNA replication. An assay of the enzyme's exonuclease activity determines that it is active on both intact and demethylated thymine nucleotides. Which of the following enzymes have the investigators most likely isolated?

A. DNA ligase
B. Telomerase
C. Primase
D. DNA topoisomerase
E. DNA polymerase I (Correct Answer)

Explanation: ***DNA polymerase I*** - **DNA polymerase I** possesses **5' to 3' exonuclease activity**, which is crucial for removing **RNA primers** (intact nucleotides) laid down by primase during DNA replication. - This 5' to 3' exonuclease activity also allows it to excise damaged DNA, including DNA containing **demethylated thymine nucleotides**. - It also has 3' to 5' exonuclease activity for proofreading. - **Key distinction:** While DNA polymerase III (the main replicative enzyme) only has 3' to 5' exonuclease activity, DNA polymerase I has **both** 3' to 5' and 5' to 3' exonuclease activities, making it essential for primer removal and DNA repair. *DNA ligase* - **DNA ligase** functions to form a **phosphodiester bond** between adjacent nucleotides to seal nicks in the DNA backbone, but it does not have exonuclease activity. - Its primary role is in joining Okazaki fragments and repairing single-strand breaks. *Telomerase* - **Telomerase** is a specialized reverse transcriptase that extends the telomeres at the ends of eukaryotic chromosomes, but is not present in prokaryotes like *E. coli*. - It uses an RNA template to synthesize DNA, and it lacks exonuclease activity. *Primase* - **Primase** is an RNA polymerase that synthesizes short **RNA primers** on the DNA template, providing a starting point for DNA synthesis. - It is involved in synthesizing primers, not in removing or excising nucleotides, and has no exonuclease activity. *DNA topoisomerase* - **DNA topoisomerases** relieve supercoiling in DNA during replication and transcription by cutting and rejoining DNA strands. - While they act on DNA, their function is to manage topological stress, and they do not exhibit exonuclease activity on nucleotides.

Question 201: A 3-year-old girl is brought to the emergency department for 2 days of abdominal pain and watery diarrhea. This morning her stool had a red tint. She and her parents visited a circus 1 week ago. The patient attends daycare. Her immunizations are up-to-date. Her temperature is 38°C (100.4°F), pulse is 140/min, and blood pressure is 80/45 mm Hg. Abdominal examination shows soft abdomen that is tender to palpation in the right lower quadrant with rebound. Stool culture grows Yersinia enterocolitica. Exposure to which of the following was the likely cause of this patient's condition?

A. Undercooked poultry
B. Home-canned food
C. Deli meats
D. Unwashed vegetables
E. Undercooked pork (Correct Answer)

Explanation: ***Undercooked pork*** - **Yersinia enterocolitica** is commonly transmitted through contaminated food, especially **undercooked pork** and **chitterlings**. - The patient's symptoms (abdominal pain, watery diarrhea with a red tint, fever) and the growth of *Yersinia enterocolitica* in stool culture are consistent with yersiniosis acquired from this source. *Undercooked poultry* - **Undercooked poultry** is a common source of **Salmonella** and **Campylobacter**, which also cause gastroenteritis symptoms. - However, the stool culture specifically identified *Yersinia enterocolitica*, making poultry less likely as the primary source in this case. *Home-canned food* - **Home-canned foods** are primarily associated with **Clostridium botulinum** and **botulism**, which presents with neurological symptoms (e.g., paralysis) rather than gastroenteritis. - *Yersinia enterocolitica* is not typically linked to home-canned food contamination. *Deli meats* - **Deli meats** are often implicated in **Listeria monocytogenes** infections or **Staphylococcus aureus** food poisoning. - While they can cause gastrointestinal symptoms, the specific pathogen *Yersinia enterocolitica* is not primarily associated with deli meats. *Unwashed vegetables* - **Unwashed vegetables** can be a source of various pathogens, including **E. coli**, **Salmonella**, and **norovirus**. - While possible, *Yersinia enterocolitica* has a stronger epidemiological link to undercooked pork products.

Question 202: A 44-year-old Caucasian male presents with a fever, recent weight loss, and a cough productive of bloody sputum. A chest X-ray and CT scan were performed, revealing cavities near the apex of his lungs. The patient is started on rifampin, isoniazid, ethambutol and pyrazinamide. Formation of the cavities in the patient's lungs is mainly mediated by:

A. NK cells
B. Apoptosis
C. B-cells
D. Toxin secretion by the bacterium
E. TH1 cells (Correct Answer)

Explanation: ***TH1 cells*** - **Mycobacterium tuberculosis** infection primarily involves a **TH1 cell-mediated immune response**, which includes macrophages, epithelioid cells, and giant cells forming **granulomas**. - The formation of **cavities** in tuberculosis is a result of **caseous necrosis** within these granulomas, driven by the intense destructive effects of TH1-driven inflammation attempting to contain the infection. *NK cells* - **Natural killer (NK) cells** play a role in early host defense against intracellular pathogens, including *Mycobacterium tuberculosis*, by producing **interferon-gamma** and directly killing infected cells. - However, they are not the primary mediators of the extensive tissue destruction and cavitation seen in advanced pulmonary tuberculosis. *Apoptosis* - **Apoptosis**, or programmed cell death, plays a complex role in tuberculosis, both in host defense (killing infected cells) and potentially in M. tuberculosis survival mechanisms. - While apoptosis contributes to cell death within granulomas and necrotic lesions, it is part of a broader immune response, not the main driving force for large-scale tissue cavitation. *B-cells* - **B-cells** are involved in the **humoral immune response**, producing antibodies that target extracellular pathogens or toxins. - While antibodies can be detected in tuberculosis, they do not play a significant role in the cell-mediated immunity required to contain intracellular *M. tuberculosis* infection or in the formation of lung cavities. *Toxin secretion by the bacterium* - Unlike many bacterial infections that cause tissue damage through **exotoxins** or **endotoxins**, *Mycobacterium tuberculosis* does not secrete potent toxins that directly cause the extensive cavitary lesions. - The destruction and cavitation are primarily due to the host's vigorous, but often dysregulated, **cell-mediated immune response**.

Question 203: A 24-year-old woman presents to her primary care doctor with a lesion on her labia. She first noticed the lesion 2 days ago. It is not painful. She denies vaginal discharge or dysuria. She has no past medical history and takes no medications. She has had 4 sexual partners in the past 8 months and uses the pull-out method as contraception. She drinks 12-16 alcoholic beverages per week and is a law student. Her temperature is 97.8°F (36.6°C), blood pressure is 121/81 mmHg, pulse is 70/min, and respirations are 16/min. On exam, she has an indurated non-tender ulcer on the left labia majora. There is no appreciable inguinal lymphadenopathy. Multiple tests are ordered and pending. This patient's condition is most likely caused by a pathogen with which of the following characteristics on microscopic examination?

A. Motile and helical-shaped bacteria (Correct Answer)
B. Rod-shaped organisms in phagocyte cytoplasm
C. Gram-negative coccobacillus with a "school of fish" appearance
D. Vaginal epithelial cells covered with bacteria
E. Gram-negative diplococci

Explanation: ***Motile and helical-shaped bacteria*** - The patient's **painless, indurated genital ulcer** (chancre) strongly suggests **primary syphilis**. - Syphilis is caused by *Treponema pallidum*, a **spirochete** characterized by its **motile, helical shape**. *Rod-shaped organisms in phagocyte cytoplasm* - This description is characteristic of **Donovan bodies**, which are found in **macrophages** in cases of **granuloma inguinale** (donovanosis). - Granuloma inguinale typically presents as **painless, beefy-red ulcers** that *bleed easily* and are *not indurated*. *Gram-negative coccobacillus with a "school of fish" appearance* - This appearance is characteristic of *Haemophilus ducreyi*, the causative agent of **chancroid**. - Chancroid typically presents with **painful, ragged ulcers** and often leads to **inguinal lymphadenopathy** (buboes), which is absent here. *Vaginal epithelial cells covered with bacteria* - This describes **clue cells**, which are characteristic of **bacterial vaginosis** (*Gardnerella vaginalis*). - Bacterial vaginosis presents with **vaginal discharge**, a **fishy odor**, and dysuria, not a solitary ulcer. *Gram-negative diplococci* - This morphology is characteristic of **Neisseria gonorrhoeae**, the causative agent of **gonorrhea**. - Gonorrhea typically causes **urethritis** with **purulent discharge** in women, or can be asymptomatic; it does not cause a solitary genital ulcer.

Question 204: A 36-year-old woman with a past medical history of diabetes comes to the emergency department for abdominal pain. She reports that a long time ago her gynecologist told her that she had “some cysts in her ovaries but not to worry about it.” The pain started last night and has progressively gotten worse. Nothing seems to make it better or worse. She denies headache, dizziness, chest pain, dyspnea, diarrhea, or constipation; she endorses nausea, dysuria for the past 3 days, and chills. Her temperature is 100.7°F (38.2°C), blood pressure is 132/94 mmHg, pulse is 104/min, and respirations are 14/min. Physical examination is significant for right lower quadrant and flank pain with voluntary guarding. What is the most likely pathophysiology of this patient’s condition?

A. Ascending infection of the urinary tract (Correct Answer)
B. Inflammation of the appendix
C. Cessation of venous drainage from the ovaries
D. Irritation of the peritoneal lining
E. Vesicoureteral reflux

Explanation: ***Ascending infection of the urinary tract*** - The patient's symptoms of **dysuria**, **flank pain**, **chills**, and **fever** along with **right lower quadrant pain** are highly suggestive of **pyelonephritis**, which is an ascending urinary tract infection. Her history of diabetes also increases her risk for UTIs. - **Voluntary guarding** in the right lower quadrant and flank further supports an inflammatory process in the kidney or surrounding structures, consistent with a severe UTI escalating to the kidneys. *Inflammation of the appendix* - While **right lower quadrant pain** is present, the key symptoms of **dysuria** and **flank pain** are not typical for appendicitis. - Appendicitis often presents with migration of pain from the periumbilical region to the right lower quadrant, and while fever can occur, it's usually not associated with prominent dysuria. *Cessation of venous drainage from the ovaries* - This describes **ovarian torsion**, which typically presents with sudden, severe, unilateral lower abdominal pain and can be associated with nausea and vomiting. - However, the presence of **dysuria**, **flank pain**, and **fever** (suggesting infection) makes ovarian torsion less likely as the primary pathology. *Irritation of the peritoneal lining* - Peritoneal irritation (peritonitis) can cause abdominal pain and guarding, but it's a general sign and not a specific diagnosis of the underlying cause. - While peritonitis could be a complication, the precise constellation of symptoms points more directly to a genitourinary infection. *Vesicoureteral reflux* - **Vesicoureteral reflux (VUR)** is the abnormal backward flow of urine from the bladder into the ureters and sometimes the kidneys. It's a predisposing factor for ascending UTIs, especially in children, rather than a direct *description of the acute pathophysiology* here. - In this acute presentation, VUR would facilitate the **ascending infection**, but it doesn't explain the acute symptoms on its own.

Question 205: A 24-year-old woman presents with fever, abdominal pain, and bloody bowel movements. She says her symptoms onset 2 days ago and have not improved. She describes the abdominal pain as moderate, cramping in character, and poorly localized. 1 week ago, she says she was on a camping trip with her friends and had barbecued chicken which she thought tasted strange. The patient denies any chills, hemoptysis, hematochezia, or similar symptoms in the past. The vital signs include: pulse 87/min and temperature 37.8°C (100.0°F). Physical examination is significant for moderate tenderness to palpation in the periumbilical region with no rebound or guarding. Stool is guaiac positive. Which of the following is a complication associated with this patient’s most likely diagnosis?

A. Reactive arthritis
B. Hemolytic uremic syndrome
C. Toxic megacolon
D. Bacteremia
E. Guillain-Barré syndrome (Correct Answer)

Explanation: ***Guillain-Barré syndrome*** - **Guillain-Barré syndrome (GBS)** is a rare but serious post-infectious autoimmune neuropathy that can be triggered by *Campylobacter jejuni* infection. - The patient's symptoms (fever, abdominal pain, bloody bowel movements after consuming undercooked chicken) are highly suggestive of **Campylobacter enteritis**, making GBS a potential complication. *Reactive arthritis* - While **reactive arthritis** can be a complication of *Campylobacter* infection, it typically involves sterile joint inflammation and is less severe than Guillain-Barré syndrome. - Its clinical presentation often includes **asymmetric oligoarthritis**, dactylitis, and enthesitis, which are not described as the primary concern here. *Hemolytic uremic syndrome* - **Hemolytic uremic syndrome (HUS)** is primarily associated with **Shiga toxin-producing *E. coli* (STEC)** infections, particularly O157:H7. - Although *Campylobacter* can rarely cause HUS, it's not the most common or direct complication compared to other options. *Toxic megacolon* - **Toxic megacolon** is a severe complication of inflammatory bowel disease or other severe colitis (e.g., *C. difficile* infection), characterized by acute dilation of the colon with systemic toxicity. - While severe *Campylobacter* infection can cause colitis, toxic megacolon is a less common and specific complication in this context. *Bacteremia* - While **bacteremia** is possible with severe gastrointestinal infections, it is a direct extension of the infection itself rather than a post-infectious immune-mediated complication like Guillain-Barré syndrome. - The question asks for a specific "complication," implying a distinct secondary condition.

Question 206: A 47-year-old woman presents to a local medical shelter while on a mission trip with her church to help rebuild homes after a hurricane. She has been experiencing severe nausea, vomiting, and diarrhea for the last 2 days and was feeling too fatigued to walk this morning. On presentation, her temperature is 99.2°F (37.3°C), blood pressure is 95/62 mmHg, pulse is 121/min, and respirations are 17/min. Physical exam reveals decreased skin turgor, and a stool sample reveals off-white watery stools. Gram stain reveals a gram-negative, comma-shaped organism that produces a toxin. Which of the following is consistent with the action of the toxin most likely involved in the development of this patient's symptoms?

A. Decreased ribosomal activity
B. Increased membrane permeability
C. Cleavage of junctional proteins
D. Increased adenylyl cyclase activity (Correct Answer)
E. Activation of receptor tyrosine kinase

Explanation: ***Increased adenylyl cyclase activity*** - The patient's symptoms (severe nausea, vomiting, diarrhea, dehydration, **hypotension**, and **tachycardia**) along with the presence of a **gram-negative, comma-shaped organism** producing an off-white watery stool are highly suggestive of **Cholera** caused by *Vibrio cholerae*. - Cholera toxin is an **AB5 toxin** that irreversibly activates **adenylyl cyclase** in intestinal epithelial cells, leading to increased intracellular cyclic AMP (cAMP) levels. This increased cAMP then causes massive secretion of chloride and bicarbonate into the intestinal lumen, followed by water, resulting in the characteristic **"rice-water stool"**. *Decreased ribosomal activity* - This mechanism is characteristic of toxins like **Shiga toxin** (produced by *Shigella dysenteriae* and enterohemorrhagic *E. coli*) and **diphtheria toxin** (produced by *Corynebacterium diphtheriae*). - These toxins inhibit protein synthesis by inactivating the 60S ribosomal subunit, which typically leads to **cytotoxicity** rather than the profuse watery diarrhea seen in cholera. *Increased membrane permeability* - Some toxins, such as **alpha-toxin** of *Clostridium perfringens* (gas gangrene) or **pore-forming toxins**, increase membrane permeability by creating pores in cell membranes. - While this can lead to cell damage and lysis, it is not the primary mechanism by which the cholera toxin causes massive fluid secretion. *Cleavage of junctional proteins* - Toxins that cleave **tight junction proteins** (e.g., *Clostridium difficile* toxins A and B) can disrupt the intestinal barrier and lead to fluid leakage. - However, the main mechanism of cholera toxin is fluid secretion due to ion channel activation rather than direct disruption of intercellular junctions. *Activation of receptor tyrosine kinase* - Activation of **receptor tyrosine kinases** is typically involved in cell growth, differentiation, and metabolism, not directly in acute, severe secretory diarrhea. - While some bacterial toxins can modulate host signaling pathways, direct activation of receptor tyrosine kinases is not the primary mechanism of action for toxins causing cholera-like symptoms.

Question 207: A 67-year-old woman comes to the physician because of fever, chills, myalgias, and joint pain 1 month after undergoing aortic prosthetic valve replacement due to high-grade aortic stenosis. She does not drink alcohol or use illicit drugs. Her temperature is 39.3°C (102.8°F). She appears weak and lethargic. Physical examination shows crackles at both lung bases and a grade 2/6, blowing diastolic murmur over the right sternal border. Laboratory studies show leukocytosis and an elevated erythrocyte sedimentation rate. The causal organism is most likely to have which of the following characteristics?

A. Beta hemolytic, bacitracin-sensitive cocci
B. Alpha hemolytic, optochin-resistant cocci
C. Catalase-negative cocci that grows in 6.5% saline
D. Novobiocin-sensitive, coagulase-negative cocci (Correct Answer)
E. Alpha hemolytic, optochin-sensitive diplococci

Explanation: ***Novobiocin-sensitive, coagulase-negative cocci*** - The patient's symptoms (fever, chills, new murmur) and recent **prosthetic valve replacement** strongly suggest **nosocomial infective endocarditis**. - **Staphylococcus epidermidis** is a common cause of prosthetic valve endocarditis, and it is a **coagulase-negative Staphylococcus** that is characteristically **novobiocin-sensitive**. *Beta hemolytic, bacitracin-sensitive cocci* - This describes **Group A Streptococcus (Streptococcus pyogenes)**, which causes pharyngitis, cellulitis, and toxic shock syndrome, but rarely infective endocarditis, particularly 1 month post-op. - While it can cause rheumatic fever (leading to valve damage), it is not a common cause of prosthetic valve endocarditis in this specific context. *Alpha hemolytic, optochin-resistant cocci* - This describes **Viridans group streptococci (e.g., Streptococcus mitis, S. sanguinis)**, which are common causes of native valve endocarditis, often following dental procedures. - However, they are typically **alpha-hemolytic** and **optochin-resistant**, not associated with prosthetic valve infections in the immediate post-operative period. *Catalase-negative cocci that grows in 6.5% saline* - This describes **Enterococci (e.g., Enterococcus faecalis, Enterococcus faecium)**. They are catalase-negative and can grow in 6.5% saline. - While enterococci can cause endocarditis, particularly in patients with genitourinary or gastrointestinal procedures, they are not the most likely cause of prosthetic valve endocarditis 1 month after surgery. *Alpha hemolytic, optochin-sensitive diplococci* - This describes **Streptococcus pneumoniae**, a common cause of pneumonia, meningitis, and otitis media. - While it can cause endocarditis, it is less common for prosthetic valve endocarditis in this setting and would typically present with more prominent respiratory symptoms.

Question 208: A 28-year-old man comes to the physician for a pre-employment examination. He has no history of serious illness and takes no medications. A screening blood test is performed in which peptides are added to the sample to stimulate in vitro production of interferon-gamma, which is then measured using an enzyme-linked immunosorbent assay. This test is most likely to be helpful in diagnosing infection with which of the following pathogens?

A. Mycobacterium tuberculosis (Correct Answer)
B. Staphylococcus aureus
C. Human immunodeficiency virus
D. Hepatitis B virus
E. Legionella pneumophila

Explanation: ***Mycobacterium tuberculosis*** - The described test, an **interferon-gamma release assay (IGRA)**, specifically measures immune response to *Mycobacterium tuberculosis* antigens. - IGRAs are used to diagnose **latent tuberculosis infection (LTBI)** by detecting T-cell mediated immunity to the bacteria. *Staphylococcus aureus* - This bacterium causes a variety of infections, but its diagnosis primarily relies on **bacterial culture** and **antigen detection**, not interferon-gamma release assays. - *Staphylococcus aureus* is a common **bacterial pathogen** with different diagnostic approaches. *Human immunodeficiency virus* - **HIV infection** is diagnosed by detecting anti-HIV antibodies, HIV RNA, or p24 antigen through tests like **ELISA** and **Western blot**, not IGRA. - The virus primarily targets **CD4+ T cells**, leading to immunodeficiency. *Hepatitis B virus* - **HBV infection** is diagnosed by detecting viral **antigens (HBsAg, HBeAg)** and **antibodies (anti-HBs, anti-HBc)** in the blood. - It is a **DNA virus** causing hepatitis. *Legionella pneumophila* - **Legionnaire's disease**, caused by *Legionella pneumophila*, is typically diagnosed via **urine antigen testing**, **sputum culture**, or **PCR**. - This bacterium is primarily associated with **respiratory tract infections**.

Question 209: A 27-year-old man who recently emigrated as a refugee from Somalia presents with fever, weight loss, fatigue, and exertional chest pain. He says his symptoms began 3 weeks ago and that his appetite has decreased and he has lost 3 kg (6.6 lb) in the last 3 weeks. He denies any history of cardiac disease. His past medical history is unremarkable. The patient admits that he has always lived in poor hygienic conditions in overcrowded quarters and in close contact with cats. His vital signs include: blood pressure 120/60 mm Hg, pulse 90/min, and temperature 38.0°C (100.4°F). Physical examination reveals generalized pallor. A cardiac examination reveals an early diastolic murmur loudest at the left third intercostal space. Abdominal examination reveals a tender and mildly enlarged spleen. Prominent axillary lymphadenopathy is noted. Laboratory investigations reveal a WBC count of 14,500/μL with 5% bands and 93% polymorphonuclear cells. An echocardiogram reveals a 5-mm vegetation on the aortic valve with moderate regurgitation. Three sets of blood cultures are taken over 24 hours followed by empiric antibiotic therapy with gentamicin and vancomycin. The blood cultures show no growth after 5 days. Following a week of empiric therapy, the patient continues to deteriorate. Which of the following would most likely confirm the diagnosis in this patient?

A. Epstein-Barr virus heterophile antibody
B. Peripheral blood smear
C. Q fever serology
D. Bartonella serology (Correct Answer)
E. HIV polymerase chain reaction

Explanation: **Bartonella serology** - The patient's history of **poor hygiene**, **cat exposure**, culture-negative endocarditis with **aortic valve vegetation**, and **axillary lymphadenopathy** are highly suggestive of **Bartonella endocarditis**. - **Bartonella henselae**, the causative agent of **cat scratch disease**, can cause indolent, culture-negative endocarditis, and serology is a key diagnostic tool. *Epstein-Barr virus heterophile antibody* - This test is primarily used to diagnose **infectious mononucleosis**, which typically presents with **pharyngitis, lymphadenopathy**, and **fatigue**. - While fatigue and lymphadenopathy are present, the patient's **fever, weight loss, exertional chest pain**, and **endocarditis** are not typical features of mononucleosis. *Peripheral blood smear* - A peripheral blood smear is used to identify **hematological abnormalities** like **anemia, leukemia**, or **parasitic infections** (e.g., malaria). - While **generalized pallor** suggests anemia, it would not directly confirm the cause of the **culture-negative endocarditis** or explain the specific constellation of symptoms. *Q fever serology* - **Q fever**, caused by **Coxiella burnetii**, can cause **culture-negative endocarditis**, particularly in patients with pre-existing valvular disease. - However, the strong history of **cat exposure** and **axillary lymphadenopathy** in this case makes **Bartonella** a more probable diagnosis given the available information. *HIV polymerase chain reaction* - **HIV infection** can lead to various opportunistic infections and systemic symptoms like **weight loss, fever**, and **lymphadenopathy**. - While HIV can predispose to endocarditis, the specific clinical picture with **cat exposure** and **culture-negative endocarditis** points more directly to **Bartonella infection** rather than primary HIV as the cause of the endocarditis.

Question 210: A 39-year-old woman presents to the emergency department with fever, cough, and shortness of breath. She reports developing flu-like symptoms 7 days ago but progressively worsened to the point where she experiences dyspnea on exertion. Her cough is accompanied by a mild amount of yellow sputum. Past medical history is notable for a previous admission to the hospital for pneumonia 4 months ago and an admission for bacteremia 6 weeks ago. She additionally has a history of IV heroin abuse, but her last use of heroin was 3 years ago. Temperature is 101.2°F (38.4°C), blood pressure is 104/70 mmHg, pulse is 102/min, and respirations are 20/min. Physical examination demonstrates coarse upper airway breath sounds over the right lower lung field. A faint 1/6 non-radiating systolic flow murmur is auscultated at the first right intercostal space. Abdominal examination is significant for moderate splenomegaly. Tenderness of the wrists and fingers is elicited on palpation, and range of motion is restricted. The patient comments that her range of motion and pain usually improve as the day goes on. Which of the following laboratory abnormalities is most likely to be found in this patient?

A. Positive HIV serology
B. Flow cytometry positive for CD11c and CD2
C. Decreased anion gap
D. Leukocytosis with left-shift (Correct Answer)
E. Neutropenia

Explanation: ***Leukocytosis with left-shift*** - The patient presents with **fever, cough, shortness of breath, mild yellow sputum, tachycardia, and a history of recurrent infections (pneumonia, bacteremia)**. These symptoms are highly suggestive of a severe ongoing bacterial infection, which typically causes an increase in **white blood cell count (leukocytosis)** and an elevated proportion of immature neutrophils (**left shift**). - The presence of **splenomegaly**, a **systolic flow murmur**, and **tenderness in the joints** (which improves with activity, possibly indicating inflammation) further supports a systemic inflammatory response, likely due to a bacterial process such as **endocarditis** (suggested by the murmur and IV drug use history) or a severe pneumonia. *Positive HIV serology* - While a history of **IV heroin abuse** is a risk factor for HIV, the patient's current presentation is more acutely indicative of a severe bacterial infection rather than directly pointing to HIV as the most likely immediate laboratory finding. Other infections could be the cause of her recurrent pneumonia admissions. - HIV infection would lead to **immunosuppression**, potentially explaining recurrent infections, but does not directly cause the acute inflammatory picture of **leukocytosis and left shift** as the primary lab abnormality in the context of her current symptoms. *Flow cytometry positive for CD11c and CD2* - This finding is characteristic of certain **lymphoproliferative disorders**, such as **hairy cell leukemia (CD11c)** or **T-cell lymphomas (CD2)**. - The patient's presentation with **acute febrile illness, respiratory symptoms, and signs of systemic inflammation** is inconsistent with the typical acute presentation of these hematologic malignancies. *Decreased anion gap* - A **decreased anion gap** is rare and usually indicates conditions like **hypoalbuminemia**, multiple myeloma (due to increased unmeasured cations such as IgG paraprotein), or bromide intoxication. - It is not typically associated with acute bacterial infections or the inflammatory response described in this patient. *Neutropenia* - **Neutropenia** (a decrease in neutrophils) would make the patient highly susceptible to infections, but her current presentation with **fever, severe respiratory symptoms, and signs of systemic inflammation** strongly points towards an active bacterial infection where the body is mounting a robust immune response, usually characterized by an *increase* in neutrophils. - While severe overwhelming infections can sometimes lead to **neutropenia** due to bone marrow exhaustion, **leukocytosis with left shift** is a more common and earlier response to acute bacterial infection.

Question 211: A 10-month-old boy is referred to the hospital because of suspected severe pneumonia. During the first month of his life, he had developed upper airway infections, bronchitis, and diarrhea. He has received all the immunizations according to his age. He failed to thrive since the age of 3 months. A month ago, he had a severe lung infection with cough, dyspnea, and diarrhea, and was unresponsive to an empiric oral macrolide. Upon admission to his local hospital, the patient has mild respiratory distress and crackles on auscultation. The temperature is 39.5°C (103.1°F), and the oxygen saturation is 95% on room air. The quantitative immunoglobulin tests show increased IgG, IgM, and IgA. The peripheral blood smear shows leukocytosis and normochromic normocytic anemia. The chloride sweat test and tuberculin test are negative. The chest X-ray reveals bilateral pneumonia. The bronchoalveolar lavage and gram stain report gram-negative bacteria with a growth of Burkholderia cepacia on culture. The laboratory results on admission are as follows: Leukocytes 36,600/mm3 Neutrophils 80% Lymphocytes 16% Eosinophils 1% Monocytes 2% Hemoglobin 7.6 g/dL Creatinine 0.8 mg/dL BUN 15 mg/dL Which of the following defects of neutrophil function is most likely responsible?

A. Phagocytosis defect
B. Absent respiratory burst (Correct Answer)
C. Leukocyte adhesion molecule deficiency
D. X-linked agammaglobulinemia
E. Lysosomal trafficking defect

Explanation: ***Absent respiratory burst*** - The recurrent infections, particularly with **catalase-positive organisms** like *Burkholderia cepacia*, in the presence of **leukocytosis** and **hypergammaglobulinemia**, strongly point to **chronic granulomatous disease (CGD)**. CGD is characterized by an absent respiratory burst, essential for killing ingested microbes. - Patients with CGD cannot produce reactive oxygen species (ROS) due to defects in **NADPH oxidase**, leading to impaired killing of certain bacteria and fungi. *Phagocytosis defect* - While phagocytosis is crucial for immune defense, a primary defect in phagocytosis itself is less likely given the patient's presentation with **leukocytosis** and **neutrophilia**, suggesting that neutrophils are present and can be recruited, but are ineffective in killing the pathogen. - Conditions with primary phagocytosis defects generally present with different clinical features or would not specifically predispose to infections with *Burkholderia cepacia* in the context of an otherwise activated immune response (high Ig levels). *Leukocyte adhesion molecule deficiency* - This condition is characterized by **recurrent bacterial infections** and **impaired wound healing**, but a key finding is persistent **leukocytosis** with a **lack of pus formation** at infection sites and delayed umbilical cord separation. - Absence of pus formation and delayed umbilical cord separation are not mentioned, and the primary failure here appears to be bacterial killing rather than migration to the infection site. *X-linked agammaglobulinemia* - This disorder is characterized by a **severe deficiency or absence of B cells** and immunoglobulins, making patients highly susceptible to bacterial infections, particularly encapsulated bacteria. - The patient's **quantitative immunoglobulin tests show increased IgG, IgM, and IgA**, which directly contradicts the immunologic hallmark of X-linked agammaglobulinemia. *Lysosomal trafficking defect* - A lysosomal trafficking defect, such as in **Chédiak-Higashi syndrome**, causes **impaired lysosome-phagosome fusion**, leading to recurrent infections with **pyogenic bacteria**, partial oculocutaneous albinism, and neurological abnormalities. - While infections are prominent, the specific pathogen *Burkholderia cepacia* and the complete clinical picture align more closely with a defect in the respiratory burst rather than lysosomal trafficking; albinism and neurological symptoms are also absent.

Question 212: A 51-year-old man presents complaining of decreased vibratory sense in his lower limbs. Physical exam reveals a widened pulse pressure and a decrescendo murmur occurring after the S2 heart sound. After further questioning, he also reports he experienced a maculopapular rash over his trunk, palms and soles many years ago that resolved on its own. In order to evaluate the suspected diagnosis, the physician FIRST tested for which of the following?

A. Agglutination of patient's serum with Proteus O antigens
B. Gram negative, oxidase positive, comma shaped bacteria growing at 42 degrees C
C. Cytoplasmic inclusions on Giemsa stain
D. Agglutination of antibodies with beef cardiolipin (Correct Answer)
E. Indirect immunofluorescence of the patient's serum and killed T. pallidum

Explanation: ***Agglutination of antibodies with beef cardiolipin*** - The patient's symptoms (decreased vibratory sense, widened pulse pressure, decrescendo murmur, history of maculopapular rash) are highly suggestive of **tertiary syphilis with neurosyphilis and cardiovascular syphilis**. - **Agglutination of antibodies with beef cardiolipin** refers to the **VDRL (Venereal Disease Research Laboratory) test**, a non-treponemal test used for initial screening of syphilis. *Agglutination of patient's serum with Proteus O antigens* - This describes the **Weil-Felix test**, which is used to diagnose **Rickettsial infections**, not syphilis. - Rickettsial infections typically present with fever, headache, and rash, but not the specific cardiovascular or neurological signs seen in this patient. *Gram negative, oxidase positive, comma shaped bacteria growing at 42 degrees C* - This describes the characteristics of **Campylobacter jejuni**, a common cause of **gastroenteritis**, which is unrelated to the described symptoms. - This bacterium is associated with food poisoning and sometimes Guillain-Barré syndrome, but not syphilis. *Cytoplasmic inclusions on Giemsa stain* - **Cytoplasmic inclusions on Giemsa stain** are characteristic of **Chlamydia trachomatis**, which causes conditions like trachoma or chlamydial infections. - While Chlamydia can cause a rash, it does not lead to the specific cardiovascular or neurological manifestations of tertiary syphilis. *Indirect immunofluorescence of the patient's serum and killed T. pallidum* - This describes the **FTA-ABS (Fluorescent Treponemal Antibody Absorption) test**, which is a **specific treponemal test** for syphilis. - While highly accurate, the FTA-ABS is typically used as a **confirmatory test** *after* a positive non-treponemal screening test (like VDRL), making VDRL the *first* test.

Question 213: A 55-year-old man presents to the physician with complaints of 5 days of watery diarrhea, fever, and bloating. He has not noticed any blood in his stool. He states that his diet has not changed recently, and his family has been spared from diarrhea symptoms despite eating the same foods that he has been cooking at home. He has no history of recent travel outside the United States. His only medication is high-dose omeprazole, which he has been taking daily for the past few months to alleviate his gastroesophageal reflux disease (GERD). Which of the following is the most appropriate initial test to work up this patient’s symptoms?

A. Stool toxin assay (Correct Answer)
B. Colonoscopy
C. Fecal occult blood test
D. Stool culture
E. Stool ova and parasite

Explanation: ***Stool toxin assay*** - The patient's presentation of **watery diarrhea** and fever, especially with a history of **high-dose omeprazole use**, strongly suggests **Clostridioides difficile infection**. - **Omeprazole** (a proton pump inhibitor) reduces stomach acid, which can disrupt the normal gut flora and increase susceptibility to *C. difficile*; a **stool toxin assay** is the most direct diagnostic test for this infection. *Colonoscopy* - While a colonoscopy can visualize pseudomembranes associated with severe *C. difficile* colitis, it is an **invasive procedure** and not the initial diagnostic test of choice for suspected infectious diarrhea. - It is usually reserved for cases with atypical presentations, suspected complications, or when other diagnostic tests are inconclusive. *Fecal occult blood test* - The patient describes **watery diarrhea** and specifically states he has **not noticed any blood in his stool**, making a fecal occult blood test unlikely to be helpful in this acute setting. - This test is primarily used for screening **colorectal cancer** or identifying chronic gastrointestinal bleeding. *Stool culture* - A stool culture primarily identifies bacterial pathogens like *Salmonella*, *Shigella*, or *Campylobacter*, which typically cause diarrheal illnesses that may include **bloody stools** or have specific epidemiological links (e.g., foodborne outbreaks). - Given the history of **omeprazole use** and the absence of blood, *C. difficile* is more likely than these common bacterial enteritides, and a stool culture does not detect *C. difficile* itself. *Stool ova and parasite* - This test is used to detect **parasitic infections** (e.g., Giardia, Cryptosporidium), which can cause watery diarrhea and bloating. - However, given the specific risk factor of **omeprazole use**, **Clostridioides difficile** infection is a more probable diagnosis, making the stool toxin assay the more appropriate initial test.

Question 214: An 81-year-old man comes to the emergency department with severe left ear pain and drainage for 3 days. He has a history of poorly-controlled type 2 diabetes mellitus. He appears uncomfortable. Physical examination of the ear shows marked periauricular erythema, exquisite tenderness on palpation, and granulation tissue in the external auditory canal. The most likely causal pathogen produces an exotoxin that acts by a mechanism most similar to a toxin produced by which of the following organisms?

A. Corynebacterium diphtheriae (Correct Answer)
B. Bacillus anthracis
C. Staphylococcus aureus
D. Bordetella pertussis
E. Shigella dysenteriae

Explanation: ***Corynebacterium diphtheriae*** - The clinical picture describes **malignant otitis externa** (MOE), likely caused by *Pseudomonas aeruginosa*, particularly in an elderly diabetic patient. Both *Pseudomonas aeruginosa* exotoxin A and *Corynebacterium diphtheriae* diphtheria toxin **inhibit protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2)**. - This shared mechanism of action makes *Corynebacterium diphtheriae* the most appropriate comparative organism based on the question's premise of exotoxin mechanism. *Bacillus anthracis* - Produces **anthrax toxin**, which consists of Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF). LF is a **zinc-dependent metalloprotease** that cleaves mitogen-activated protein kinase kinase (MAPKK) family proteins. - This mechanism is distinct from the ADP-ribosylation of EF-2. *Staphylococcus aureus* - Produces several toxins, including **toxic shock syndrome toxin-1 (TSST-1)** and **exfoliatin**, which act as **superantigens** or **proteases** respectively. - These mechanisms differ from the ADP-ribosylation of EF-2. *Bordetella pertussis* - Produces **pertussis toxin**, which **ADP-ribosylates Gi proteins**, leading to increased cAMP levels by disinhibiting adenylate cyclase. - This is a different target and mechanism compared to the ADP-ribosylation of EF-2. *Shigella dysenteriae* - Produces **Shiga toxin**, which is an **N-glycosidase** that cleaves the adenine residue from the 28S rRNA of the 60S ribosomal subunit, thereby **inhibiting protein synthesis**. - While it inhibits protein synthesis, the specific mechanism is different from ADP-ribosylation of EF-2.

Question 215: A 33-year-old woman comes to the emergency department because of a 3-day history of lower abdominal pain and severe burning with urination. Two years ago, she was diagnosed with cervical cancer and was successfully treated with a combination of radiation and chemotherapy. She has systemic lupus erythematosus and finished a course of cyclophosphamide 3 weeks ago. She is sexually active with multiple male and female partners and uses a diaphragm for contraception. She has smoked two packs of cigarettes daily for 12 years. Current medication includes hydroxychloroquine. Her temperature is 36.6°C (97.9°F), pulse is 84/min, and blood pressure is 136/84 mm Hg. The abdomen is soft and there is tenderness to palpation over the pelvic region. Laboratory studies show: Hemoglobin 13.1 g/dL Leukocyte count 7,400/mm3 Platelet count 210,000/mm3 Urine pH 7 WBC 62/hpf RBC 12/hpf Protein negative Nitrites positive Which of the following is the most likely underlying mechanism of this patient's condition?

A. Ascending infection (Correct Answer)
B. Hematogenous spread of infection
C. Radiation-induced inflammation
D. Neural hypersensitivity
E. Sexually transmitted infection

Explanation: ***Ascending infection*** - The combination of **dysuria**, **pelvic tenderness**, and specific urine findings (WBCs 62/hpf, RBCs 12/hpf, positive nitrites) are highly indicative of a **urinary tract infection (UTI)**. UTIs typically arise from bacteria ascending the urethra into the bladder. - Factors like a **diaphragm for contraception** and **sexual activity with multiple partners** increase the risk of UTIs due to bacterial translocation and urethral irritation. *Hematogenous spread of infection* - While possible in severely immunocompromised patients, **hematogenous spread** to the urinary tract is less common for typical UTIs and is usually associated with systemic symptoms or a distant primary infection source. - The patient's symptoms are localized to the urinary tract, and there's no evidence of a widespread systemic infection or a primary source from which bacteria would hematogenously spread to the bladder. *Radiation-induced inflammation* - **Radiation cystitis** can cause similar symptoms (dysuria, hematuria) but typically presents as **sterile pyuria** (WBCs without bacteria, though positive nitrites here suggest bacterial presence). It would also be expected to occur closer to the radiation treatment or in chronic cases, not as an acute presentation 2 years post-treatment without other contributing factors. - The presence of **nitrites** in the urine strongly suggests bacterial infection, which is not characteristic of radiation-induced inflammation alone. *Neural hypersensitivity* - **Neural hypersensitivity** can cause chronic pelvic pain and urgency but does not typically present with acute onset dysuria, significant pyuria, and positive nitrites as seen in this patient. - This mechanism is more associated with conditions like **interstitial cystitis/bladder pain syndrome**, which involves chronic pain rather than acute infection signs. *Sexually transmitted infection* - While the patient is sexually active with multiple partners, which increases the risk for STIs, common STIs like **chlamydia** or **gonorrhea** typically cause cervicitis or urethritis and often do not lead to the high WBC count and positive nitrites in the urine characteristic of a bacterial UTI. - Although STIs can present with dysuria, the full picture of **pyuria**, **hematuria**, and **positive nitrites** points more strongly to a bacterial UTI than an uncomplicated STI.

Question 216: An 11-year-old boy is brought to the emergency department by his parents with a 2-day history of fever, malaise, and productive cough. On presentation, he is found to be very weak and is having difficulty breathing. His past medical history is significant for multiple prior infections requiring hospitalization including otitis media, upper respiratory infections, pneumonia, and sinusitis. His family history is also significant for a maternal uncle who died of an infection as a child. Lab findings include decreased levels of IgG, IgM, IgA, and plasma cells with normal levels of CD4 positive cells. The protein that is most likely defective in this patient has which of the following functions?

A. Lysosomal trafficking
B. Actin polymerization
C. Protein phosphorylation (Correct Answer)
D. Nucleotide salvage
E. Autoimmune regulation

Explanation: ***Protein phosphorylation*** - The patient's history of recurrent infections, decreased immunoglobulins (IgG, IgM, IgA), and normal CD4 counts points towards **X-linked agammaglobulinemia (XLA)**. - XLA is caused by a defect in **Bruton's tyrosine kinase (BTK)**, which is essential for B-cell maturation and immunoglobulin production, and its function involves **protein phosphorylation**. *Lysosomal trafficking* - Defects in lysosomal trafficking are characteristic of disorders like **Chédiak-Higashi syndrome**, which involves impaired phagocyte function and typically presents with **albinism**, **neuropathy**, and recurrent pyogenic infections, not hypogammaglobulinemia. - This condition affects **granule formation** and movement within cells, leading to dysfunctional lysosomes. *Actin polymerization* - Defects in actin polymerization are associated with disorders affecting **cytoskeletal integrity** and cell migration, such as **Wiskott-Aldrich syndrome**. - Wiskott-Aldrich syndrome typically presents with **thrombocytopenia**, **eczema**, and immunodeficiency, which is different from the presentation of generalized hypogammaglobulinemia seen here. *Nucleotide salvage* - Defects in the nucleotide salvage pathway, such as **adenosine deaminase (ADA) deficiency** or **purine nucleoside phosphorylase (PNP) deficiency**, lead to Severe Combined Immunodeficiency (SCID). - SCID typically results in severe reduction of both T and B cells, whereas this patient has normal CD4 counts and primarily a B-cell defect. *Autoimmune regulation* - Defects in autoimmune regulation are seen in conditions like **Autoimmune Polyendocrine Syndrome (APS)**, or **IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-linked)**. - These conditions primarily involve **dysregulated immune responses** leading to autoimmunity, rather than a primary defect in humoral immunity as suggested by the low immunoglobulin levels.

Question 217: A 17-year-old boy comes to the emergency department because of a 3-day history of pain in his left wrist. That morning the pain increased and he started to have chills and malaise. Last week he had self-resolving left knee pain. He is otherwise healthy and has not had any trauma to the wrist. He recently returned from a camping trip to Minnesota. He is sexually active with one female partner, who uses a diaphragm for contraception. His temperature is 37.7°C (99.9°F). Examination shows several painless violaceous vesiculopustular lesions on the dorsum of both wrists and hands; two lesions are present on the left palm. There is swelling and erythema of the left wrist with severe tenderness to palpation and passive movement. Which of the following is the most likely diagnosis?

A. Reactive arthritis
B. Disseminated gonococcal infection (Correct Answer)
C. Systemic lupus erythematosus
D. Lyme arthritis
E. Acute rheumatic fever

Explanation: ***Disseminated gonococcal infection*** - The combination of **migratory polyarthralgia** (self-resolving knee pain followed by wrist pain), **tenosynovitis** (swelling, erythema, and tenderness of the wrist), and **painless violaceous vesiculopustular lesions** on the extremities is characteristic of disseminated gonococcal infection. - The patient's age, sexual activity, and lack of trauma, along with the systemic symptoms (chills, malaise, low-grade fever), further support this diagnosis. *Reactive arthritis* - While reactive arthritis can cause **migratory arthritis**, it typically presents with **inflammatory arthritis**, **enthesitis**, and often involves genitourinary or gastrointestinal symptoms. - It does not usually present with the distinctive **dermatological manifestations** (vesiculopustular lesions) seen in this patient. *Systemic lupus erythematosus* - SLE can cause **arthralgia** and diverse dermatological manifestations, but the characteristic rash is often a **malar rash** or discoid lesions, not the vesiculopustular lesions described. - Significant fever, malaise, or specific serological markers like **ANA** would further differentiate it, which are not mentioned here. *Lyme arthritis* - Lyme arthritis typically presents with **monoarticular or oligoarticular arthritis**, often affecting large joints like the knee, and may be preceded by an **erythema chronicum migrans** rash. - The presence of **vesiculopustular skin lesions** and tenosynovitis are not typical features of Lyme arthritis. *Acute rheumatic fever* - This condition follows a **Streptococcus pyogenes infection** and is characterized by migratory polyarthritis and carditis, but it usually presents in younger children. - It does not typically include the **violaceous vesiculopustular lesions** or significant tenosynovitis observed in this patient.

Question 218: On the 4th day of hospital admission due to pneumonia, a 69-year-old woman develops non-bloody diarrhea and abdominal pain. She is currently treated with ceftriaxone. Despite the resolution of fever after the first 2 days of admission, her temperature is now 38.5°C (101.3°F). On physical examination, she has mild generalized abdominal tenderness without abdominal guarding or rebound tenderness. Laboratory studies show re-elevation of leukocyte counts. Ceftriaxone is discontinued. Given the most likely diagnosis in this patient, which of the following is the most sensitive test?

A. Nucleic acid amplification test (Correct Answer)
B. Stool culture for bacterial isolation and toxin presence
C. Enzyme immunoassay glutamate dehydrogenase
D. Gram stain of stool sample
E. Endoscopy

Explanation: ***Nucleic acid amplification test*** - **NAAT** (PCR) for *C. difficile* toxin genes is the most **sensitive** and specific test for routine clinical diagnosis of *C. difficile* infection. - It detects the **DNA** of toxin-producing *C. difficile* (tcdB gene) and is highly reliable even with low bacterial loads. - NAAT has become the **gold standard** in most clinical settings due to its rapid turnaround time (hours) and excellent sensitivity (~90-95%) and specificity (~95%). *Enzyme immunoassay glutamate dehydrogenase* - **EIA GDH** detects an antigen common to all *C. difficile* strains (both toxin-producing and non-toxin-producing). - While it has **high sensitivity** (~85-95%), it has **low specificity** and requires confirmation with a toxin test or NAAT, as it cannot distinguish between toxigenic and non-toxigenic strains. - Often used as part of a **two-step algorithm** for screening. *Gram stain of stool sample* - A **Gram stain** of stool is generally not helpful for diagnosing *C. difficile* infection. - It would show a mix of **gut flora** and would not specifically identify *C. difficile* or its toxins. *Stool culture for bacterial isolation and toxin presence* - **Stool culture** for *C. difficile* is technically the most sensitive method (~95-100%) but does not differentiate toxin-producing from non-toxin-producing strains without subsequent **toxin testing**. - It is also **time-consuming** (2-3 days) and labor-intensive, making it impractical for routine clinical diagnosis. - Primarily used for **research** or **epidemiological typing**. *Endoscopy* - **Endoscopy** with visualization of **pseudomembranes** is highly specific for severe *C. difficile* infection. - However, it is an **invasive procedure**, not sensitive for mild-to-moderate disease, and is usually reserved for cases where diagnosis is unclear or severe complications (toxic megacolon, fulminant colitis) are suspected.

Question 219: A 47-year-old man comes to the physician because of abdominal pain and foul-smelling, watery diarrhea for several days. He has not had nausea, vomiting, or blood in the stool. He has a history of alcohol use disorder and recently completed a 7-day course of clindamycin for pneumonia. He has not traveled out of the United States. Which of the following toxins is most likely to be involved in the pathogenesis of this patient's symptoms?

A. Cereulide toxin
B. Cholera toxin
C. Clostridioides difficile cytotoxin (Correct Answer)
D. Shiga toxin
E. Alpha toxin

Explanation: ***Clostridioides difficile cytotoxin*** - The patient's history of recent **clindamycin** use, followed by **abdominal pain** and **foul-smelling, watery diarrhea**, is highly suggestive of *Clostridioides difficile* infection. - *C. difficile* produces **cytotoxin (TcdB)** and **enterotoxin (TcdA)**, which lead to colitis and diarrhea, often after antimicrobial therapy. *Cereulide toxin* - This preformed toxin is produced by *Bacillus cereus* and typically causes a **short-incubation** emetic type of food poisoning, characterized by **nausea and vomiting**. - The patient's symptoms are primarily diarrhea, and nausea/vomiting are absent, making this less likely. *Cholera toxin* - Produced by *Vibrio cholerae*, this toxin causes profuse, **"rice-water" diarrhea** with rapid dehydration. - The patient has not traveled to endemic areas, and there is no mention of the characteristic severe dehydration or "rice-water" stool. *Shiga toxin* - This toxin, produced by *Shigella dysenteriae* and enterohemorrhagic *E. coli* (EHEC), typically causes **bloody diarrhea** and can lead to **hemolytic uremic syndrome (HUS)**. - The patient's diarrhea is watery and explicitly stated to be without blood, ruling out Shiga toxin as the cause. *Alpha toxin* - This toxin is produced by *Clostridium perfringens* and is primarily associated with **gas gangrene** (myonecrosis) and some forms of food poisoning. - While *C. perfringens* can cause diarrhea, it's typically mild and self-limiting, and the clinical picture in this patient, especially with recent antibiotic use, points more strongly to *C. difficile*.

Question 220: A 58-year-old female, being treated on the medical floor for community-acquired pneumonia with levofloxacin, develops watery diarrhea. She reports at least 9 episodes of diarrhea within the last two days, with lower abdominal discomfort and cramping. Her temperature is 98.6° F (37° C), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 122/98 mm Hg. Her physical examination is unremarkable. Laboratory testing shows: Hb% 13 gm/dL Total count (WBC): 13,400/mm3 Differential count: Neutrophils: 80% Lymphocytes: 15% Monocytes: 5% ESR: 33 mm/hr What is the most likely diagnosis?

A. Ulcerative colitis
B. C. difficile colitis (Correct Answer)
C. Osmotic diarrhea
D. Giardiasis
E. Irritable bowel syndrome

Explanation: ***C. difficile colitis*** - The patient's recent **antibiotic use (levofloxacin)**, followed by the development of **watery diarrhea** (9 episodes in 2 days) with abdominal cramping, is highly suggestive of *Clostridioides difficile* infection. - The elevated **WBC count (13,400/mm3)** and **ESR (33 mm/hr)** indicate an inflammatory response, which is common in *C. difficile* colitis. *Ulcerative colitis* - Ulcerative colitis typically presents with **bloody diarrhea**, abdominal pain, and tenesmus, usually with a more chronic or relapsing course, which is not described. - While it can manifest with flares, the direct temporal relationship with **antibiotic use** and the lack of bloody stools make it less likely. *Osmotic diarrhea* - Osmotic diarrhea is often related to the ingestion of **non-absorbable substances** (e.g., lactulose, sorbitol) or malabsorption and generally resolves with fasting. - It is not typically associated with a significant **inflammatory response** (elevated WBC, ESR) or a clear link to recent antibiotic use. *Giardiasis* - Giardiasis is a parasitic infection that causes **protozoal diarrhea**, often characterized by foul-smelling, fatty stools, flatulence, and abdominal cramps. - It is usually acquired from contaminated water and typically doesn't follow **antibiotic therapy** in this manner. *Irritable bowel syndrome* - IBS is a functional gastrointestinal disorder characterized by **chronic abdominal pain** and altered bowel habits (diarrhea, constipation, or both) in the absence of structural or biochemical abnormalities. - It does not present as an acute, severe diarrheal illness linked to **antibiotic use** with systemic inflammatory markers.

Question 221: A 45-year-old male presents to the emergency room complaining of severe diarrhea. He recently returned from a business trip to Bangladesh. Since returning, he has experienced several loose bloody stools per day that are accompanied by abdominal cramping and occasional nausea and vomiting. His temperature is 101.7°F (38.7°C), blood pressure is 100/60 mmHg, pulse is 120/min, and respirations are 20/min. On examination, he demonstrates mild tenderness to palpation throughout his abdomen, delayed capillary refill, and dry mucus membranes. Results from a stool sample and subsequent stool culture are pending. What is the mechanism of action of the toxin elaborated by the pathogen responsible for this patient’s current condition?

A. ADP-ribosylation of a G protein
B. ADP-ribosylation of elongation factor 2
C. Phospholipid degradation
D. Stimulation of guanylyl cyclase
E. Inhibition of 60S ribosomal subunit (Correct Answer)

Explanation: ***Inhibition of 60S ribosomal subunit*** - The patient's symptoms of **bloody diarrhea**, fever, and travel history to **Bangladesh** are highly suggestive of **Shigella dysenteriae** infection. - **Shiga toxin**, produced by *Shigella dysenteriae*, acts by **cleaving the 28S rRNA** of the **60S ribosomal subunit**, which **inhibits protein synthesis** and leads to cell death. *ADP-ribosylation of a G protein* - This mechanism is characteristic of **cholera toxin** (from *Vibrio cholerae*) and **heat-labile enterotoxin** (from *E. coli*), which cause **watery diarrhea** by activating adenylate cyclase. - These toxins typically lead to **profuse watery diarrhea** without significant bloody stools or severe inflammatory response. *ADP-ribosylation of elongation factor 2* - This is the mechanism of action of **diphtheria toxin** (from *Corynebacterium diphtheriae*) and **exotoxin A** (from *Pseudomonas aeruginosa*). - These toxins cause **tissue necrosis** and systemic effects, not typically presenting as acute bloody diarrhea. *Phospholipid degradation* - **Alpha toxin** (from *Clostridium perfringens*) and some **hemolysins** act through phospholipid degradation, causing cell lysis and tissue damage. - While *C. perfringens* can cause food poisoning, it typically results in **watery diarrhea** and abdominal cramps without the prominent bloody stools seen here. *Stimulation of guanylyl cyclase* - This mechanism is employed by **heat-stable enterotoxin** from *E. coli*, which activates guanylyl cyclase, increasing intracellular cGMP and promoting fluid secretion. - Like cholera toxin, this typically leads to **watery, non-bloody diarrhea**.

Question 222: A 67-year-old man presents to the physician with profuse watery diarrhea along with fever and crampy abdominal pain. He has been taking an antibiotic course of cefixime for about a week to treat a respiratory tract infection. At the doctor’s office, his pulse is 112/min, the blood pressure is 100/66 mm Hg, the respirations are 22/min, and the temperature is 38.9°C (102.0°F). His oral mucosa appears dry and his abdomen is soft with vague diffuse tenderness. A digital rectal examination is normal. Laboratory studies show: Hemoglobin 11.1 g/dL Hematocrit 33% Total leucocyte count 16,000/mm3 Serum lactate 0.9 mmol/L Serum creatinine 1.1 mg/dL What is most likely to confirm the diagnosis?

A. Identification of C. difficile toxin in stool (Correct Answer)
B. Colonoscopy
C. CT scan of the abdomen
D. Abdominal X-ray
E. Stool culture

Explanation: ***Identification of C. difficile toxin in stool*** - The patient's presentation with **profuse watery diarrhea**, fever, and abdominal pain following a course of **antibiotics (cefixime)** is highly suggestive of *Clostridioides difficile* infection (CDI). - **Detecting *C. difficile* toxins A or B in stool** is the most reliable and definitive method to confirm the diagnosis of CDI. *Colonoscopy* - While colonoscopy can reveal **pseudomembranes** characteristic of severe *C. difficile* colitis, it is an **invasive procedure** and not the primary diagnostic tool. - Endoscopy is typically reserved for cases where the diagnosis is uncertain despite positive toxin tests, or to assess severity when there is concern for complications like **toxic megacolon**. *CT scan of the abdomen* - A CT scan may show signs of colitis, such as **bowel wall thickening**, but it cannot definitively diagnose *C. difficile* infection. - It is more useful for identifying **complications** like toxic megacolon, bowel perforation, or ascites, rather than initial diagnosis. *Abdominal X-ray* - An abdominal X-ray is primarily used to look for signs of **toxic megacolon** or **bowel perforation** (e.g., free air under the diaphragm). - It is **not diagnostic for *C. difficile* infection itself** and would not confirm the presence of the pathogen or its toxins. *Stool culture* - A stool culture for common bacterial pathogens would likely be **negative** in a case of *C. difficile* infection, as *C. difficile* is a distinct entity. - While *C. difficile* can be cultured, **toxin detection** is preferred for diagnosis because not all *C. difficile* strains produce toxins, and asymptomatic carriage can occur.

Question 223: A 2-year-old boy is brought to the emergency department by his parents after they found him to be lethargic and febrile. His current symptoms started 1 week ago and initially consisted of a sore throat and a runny nose. He subsequently developed a fever and productive cough that has become worse over time. Notably, this patient has previously presented with pneumonia and gastroenteritis 8 times since he was born. On presentation, the patient's temperature is 103°F (39.4°C), blood pressure is 90/50 mmHg, pulse is 152/min, and respirations are 38/min. Based on clinical suspicion, an antibody panel is obtained and the results show low levels of IgG and IgA relative to the level of IgM. The expression of which of the following genes is most likely abnormal in this patient?

A. STAT3
B. LYST
C. NADPH oxidase
D. CD40L (Correct Answer)
E. CD18

Explanation: ***CD40L*** - This patient's history of **recurrent infections** (pneumonia and gastroenteritis), fever, and persistent productive cough, coupled with **low IgG and IgA levels** but **normal or elevated IgM**, is characteristic of **hyper-IgM syndrome**. - **Hyper-IgM syndrome** is most commonly caused by a genetic defect in **CD40L (CD154)** on T cells, which is essential for B cell class switching from IgM to other immunoglobulin isotopes (e.g., IgG, IgA, IgE). *STAT3* - Abnormalities in **STAT3** are associated with **hyper-IgE syndrome (Job's syndrome)**, characterized by recurrent skin abscesses, eczema, facial dysmorphism, and very high IgE levels, none of which are consistent with this patient's presentation. - While it involves immune dysregulation and infections, the **immunoglobulin profile** (low IgG/IgA, normal/high IgM) does not fit hyper-IgE syndrome. *LYST* - Defects in the **LYST gene** cause **Chediak-Higashi syndrome**, characterized by partial oculocutaneous albinism, recurrent pyogenic infections, peripheral neuropathy, and giant lysosomes in phagocytes. - The presented symptoms and **immunoglobulin abnormalities** do not align with the features of Chediak-Higashi syndrome. *NADPH oxidase* - Deficiencies in **NADPH oxidase** lead to **chronic granulomatous disease (CGD)**, where phagocytes cannot produce reactive oxygen species to kill certain microbes, resulting in recurrent severe bacterial and fungal infections and granuloma formation. - While recurrent infections are present, the **humoral immunity profile** (low IgG/IgA, high IgM) is not typical for CGD. *CD18* - Defects in **CD18**, a component of the **beta-2 integrin LFA-1**, cause **leukocyte adhesion deficiency (LAD)**. - LAD presents with recurrent bacterial infections, impaired wound healing, and a characteristic **leukocytosis** with **neutrophilia** due to the inability of leukocytes to extravasate to sites of infection. The immunoglobulin profile described is not associated with LAD.

Question 224: A 4-year-old boy is brought to the emergency department by his parents. He is lethargic and confused and has a severe headache, vomiting, and a high-grade fever since earlier that day. His mother reports that the child was doing well until 2 days ago when he developed a fever and green nasal discharge. The patient has a history of neonatal sepsis, meningococcemia at 18 months of age, and pneumococcal pneumonia at 2 and 3 years of age. His scheduled vaccinations are up to date. His blood pressure is 70/50 mm Hg, heart rate is 120/min, respiratory rate is 22/min, and temperature is 39.3°C (102.4°F). On examination, the child is lethargic and his skin is pale, with several petechiae over his buttocks. There is a purulent nasal discharge from both nostrils. The lungs are clear to auscultation bilaterally. Heart sounds are normal. There is marked neck rigidity. Cerebrospinal fluid analysis shows the following results: Opening pressure 100 mm H2O Appearance cloudy Protein 500 mg/dL (5 g/L) White blood cells 2500/μL (polymorphonuclear predominance) Glucose 31 mg/dL (1.7 mmol/L) Culture positive for N. meningitidis Which of the following immunological processes is most likely to be impaired in this child?

A. Formation of C5-9 complex (Correct Answer)
B. Production of IL-2 by Th1 cells
C. Oxidative burst in macrophages
D. Activation of TCRs by MHC-II
E. Cleavage of C2 component of complement into C2a and C2b

Explanation: ***Formation of C5-9 complex*** - The recurrent infections with **encapsulated bacteria** like *N. meningitidis* and *S. pneumoniae*, along with recurrent **meningococcemia**, strongly suggest a deficiency in the **terminal complement pathway (C5-C9)**, which is crucial for forming the **membrane attack complex (MAC)**. - The MAC is essential for **lysing gram-negative bacteria** such as *N. meningitidis*, and a defect here increases susceptibility to recurrent infections by this pathogen. *Production of IL-2 by Th1 cells* - **Interleukin-2 (IL-2)** is vital for **T-cell proliferation** and differentiation, particularly of **cytotoxic T lymphocytes (CTLs)**, important for viral and intracellular bacterial infections. - While important for overall immune function, a deficiency in IL-2 production does not specifically or primarily explain recurrent encapsulated bacterial infections and meningococcemia. *Oxidative burst in macrophages* - The **oxidative burst** is critical for phagocytic cells (neutrophils and macrophages) to kill ingested pathogens, especially **catalase-positive bacteria** and fungi. - A defect here (e.g., in **chronic granulomatous disease**) leads to recurrent severe bacterial and fungal infections, typically forming granulomas, which is not the specific pattern observed with recurrent meningococcal disease. *Activation of TCRs by MHC-II* - **T-cell receptor (TCR)** activation by **MHC-II** is central to **CD4+ helper T cell** function, essential for coordinating immune responses and antibody production. - While critical for adaptive immunity, a defect here would lead to broader immune deficiencies, including impaired antibody responses and susceptibility to various pathogens, but does not preferentially predispose to recurrent *N. meningitidis* infections like terminal complement deficiency. *Cleavage of C2 component of complement into C2a and C2b* - The cleavage of **C2** is part of the **classical and lectin complement pathways**, which are important for opsonization and inflammation. - **C2 deficiency** is often associated with recurrent pyogenic infections and autoimmune diseases (e.g., SLE), but not specifically with recurrent infections by *N. meningitidis*, which is more characteristic of **terminal complement component deficiencies**.

Question 225: A 6-year-old male who recently immigrated to the United States from Asia is admitted to the hospital with dyspnea. Physical exam reveals a gray pseudomembrane in the patient's oropharynx along with lymphadenopathy. The patient develops myocarditis and expires on hospital day 5. Which of the following would have prevented this patient's presentation and decline?

A. Increased CD4+ T cell count
B. Circulating IgG against AB exotoxin (Correct Answer)
C. Increased IgM preventing bacterial invasion
D. Improved IgE release from mast cells
E. Secretory IgA against viral proteins

Explanation: ***Circulating IgG against AB exotoxin*** - This clinical presentation (gray pseudomembrane, dyspnea, lymphadenopathy, and myocarditis) is classic for **diphtheria**, caused by **Corynebacterium diphtheriae**. - The severe manifestations, including myocarditis, are due to the **diphtheria exotoxin**, an AB toxin. **Circulating IgG antibodies** against this toxin would neutralize it, preventing its toxic effects and the ensuing severe disease. *Increased CD4+ T cell count* - While T cells are crucial for overall immune function, diphtheria's pathogenicity is primarily mediated by its **exotoxin**, not directly by bacterial invasion or intracellular survival requiring a strong cellular immune response. - An increased CD4+ T cell count alone would not directly neutralize the circulating diphtheria toxin, which is the key to preventing the severe systemic complications like myocarditis. *Increased IgM preventing bacterial invasion* - IgM antibodies are important in the **primary immune response** and can agglutinate bacteria, but they are generally less effective at neutralizing toxins compared to IgG. - The critical determinant of severe diphtheria is the **systemic spread of the exotoxin**, not simply bacterial invasion, and IgM would be less effective in preventing toxin-mediated damage. *Improved IgE release from mast cells* - IgE antibodies are primarily involved in **allergic reactions** and defense against parasites. - They play no significant role in immunity against bacterial infections like diphtheria or in neutralizing bacterial exotoxins. *Secretory IgA against viral proteins* - Secretory IgA primarily functions in **mucosal immunity** to prevent the adherence and invasion of pathogens at mucosal surfaces. - It is effective against **viral and bacterial pathogens** at mucosal sites but would not prevent the systemic effects of a bacterial exotoxin that has already been absorbed, nor would it specifically target viral proteins in a bacterial infection.

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