MD Infectious Disease and Microbiology 2

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?

Accepted Answer

Cryptococcus neoformans

Answer

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?

Accepted Answer

Shiga toxin

Answer

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?

Accepted Answer

P. falciparum

Answer

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?

Accepted Answer

Disrupting intestinal epithelial tight junctions via Ca2+ signaling

Answer

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?

Accepted Answer

Fournier's gangrene (necrotizing fasciitis)

Answer

Fournier's gangrene is a polymicrobial necrotizing fasciitis of the perineum and genitalia, often involving anaerobes producing gas, requiring emergent surgical debridement.

MD - Doctor of Medicine Practice Test

MD - Doctor of Medicine Practice Test

MD Infectious Disease and Microbiology 2