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?
Q2
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?
Q3
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?
Q4
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?
Q5
Which of the following events is likely to occur in the germinal center?
Q6
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?
Q7
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?
Q8
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?
Q9
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.
