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?
Q122
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?
Q123
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?
Q124
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?
Q125
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?
Q126
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?
Q127
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?
Q128
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?
Q129
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?
Q130
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?
Bacteria US Medical PG Practice Questions and MCQs
Question 121: 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 122: 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 123: 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 124: 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 125: 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 126: 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 127: 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 128: 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 129: 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 130: 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.
