MD Infectious Disease and Microbiology 2 — Questions and Answers
Question 1: A 35-year-old HIV-positive patient with a CD4 count of 50 cells/μL presents with headache and fever. CSF India ink preparation reveals encapsulated yeast. Which organism is responsible?
- Histoplasma capsulatum
- Cryptococcus neoformans (Correct answer)
- Candida albicans
- Aspergillus fumigatus
Correct answer: Cryptococcus neoformans
Cryptococcus neoformans causes meningitis in severely immunocompromised patients and is identified by India ink staining showing a large polysaccharide capsule.
Question 2: A patient develops bloody diarrhea after eating undercooked hamburger at a barbecue. Stool culture grows E. coli that does not ferment sorbitol. Which toxin is most responsible for hemolytic uremic syndrome in this setting?
- Heat-labile toxin
- Heat-stable toxin
- Shiga toxin (Correct answer)
- Cholera toxin
Correct answer: Shiga toxin
E. coli O157:H7 produces Shiga toxin (Stx1/Stx2) which damages renal endothelium, leading to hemolytic uremic syndrome characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury.
Question 3: A 28-year-old woman returns from Southeast Asia with cyclic fevers every 72 hours, severe anemia, and splenomegaly. Peripheral blood smear shows ring forms and banana-shaped gametocytes. Which Plasmodium species is most likely?
- P. vivax
- P. ovale
- P. malariae
- P. falciparum (Correct answer)
Correct answer: P. falciparum
P. falciparum causes 48-hour (tertian) fevers and is the only species producing banana-shaped (crescent) gametocytes; it causes the most severe malaria with high parasitemia.
Question 4: An infant develops profuse watery diarrhea after rotavirus infection. The rotavirus enterotoxin NSP4 primarily causes diarrhea by which mechanism?
- Inhibiting adenylyl cyclase
- Activating guanylyl cyclase
- Disrupting intestinal epithelial tight junctions via Ca2+ signaling (Correct answer)
- Blocking Na+/K+ ATPase
Correct answer: Disrupting intestinal epithelial tight junctions via Ca2+ signaling
Rotavirus NSP4 acts as a viral enterotoxin that mobilizes intracellular calcium, disrupting tight junction integrity and triggering chloride secretion independent of cAMP or cGMP pathways.
Question 5: A 60-year-old diabetic presents with painful, rapidly spreading erythema of the scrotum and perineum with crepitus on palpation. CT shows gas in the soft tissues. Which condition is this?
- Cellulitis due to Staphylococcus aureus
- Fournier's gangrene (necrotizing fasciitis) (Correct answer)
- Erysipelas
- Bartholin gland abscess
Correct answer: Fournier's gangrene (necrotizing fasciitis)
Fournier's gangrene is a polymicrobial necrotizing fasciitis of the perineum and genitalia, often involving anaerobes producing gas, requiring emergent surgical debridement.
Question 6: A 45-year-old cave explorer in Ohio develops mediastinal widening on CXR and hilar adenopathy weeks after his trip. Urinary antigen test is positive. Which organism is most likely?
- Coccidioides immitis
- Blastomyces dermatitidis
- Histoplasma capsulatum (Correct answer)
- Sporothrix schenckii
Correct answer: Histoplasma capsulatum
Histoplasma capsulatum is endemic to the Ohio and Mississippi River valleys and is found in soil enriched by bat or bird droppings; the urinary antigen test is the most sensitive diagnostic tool.
Question 7: A patient on broad-spectrum antibiotics develops profuse watery diarrhea. Stool toxin assay is positive for toxin B. Which antibiotic is first-line treatment?
- Ciprofloxacin
- Vancomycin (oral) (Correct answer)
- Metronidazole (IV)
- Rifaximin
Correct answer: Vancomycin (oral)
For non-severe Clostridioides difficile infection, oral vancomycin (or fidaxomicin) is now preferred over metronidazole per current IDSA guidelines due to superior cure rates.
MD Infectious Disease and Microbiology 2
A 35-year-old HIV-positive patient with a CD4 count of 50 cells/μL presents with headache and fever.
CSF India ink preparation reveals encapsulated yeast.
Which organism is responsible?