Bacteriology USMLE Step 1 Practice Question
A 28-year-old man presents to the emergency department 6 hours after eating raw oysters at a seafood restaurant. He reports acute onset of profuse, watery diarrhea and severe cramping abdominal pain. Vital signs are BP 110/68 mmHg, HR 108 bpm, RR 20/min, temperature 37.2°C. Physical examination reveals moderate dehydration with dry mucous membranes but normal skin turgor. Stool microscopy is negative for blood and WBCs. Stool culture grows oxidase-positive, curved gram-negative rods that require NaCl for growth. Serum potassium is 3.2 mEq/L. Which of the following virulence factors is primarily responsible for the massive secretory diarrhea in this patient?
Answer choices
- ALipopolysaccharide (LPS) endotoxin causing mucosal invasion and inflammation
- BCholera toxin (ADP-ribosyltransferase) that increases intestinal cAMP and chloride secretionCorrect answer
- CThermostable direct hemolysin (TDH) causing cytolysis of intestinal epithelial cells
- DInvasion plasmid antigen (Ipa) mediating bacterial translocation across the epithelium
- EShiga toxin-like cytotoxin inhibiting protein synthesis in host cells
- FType III secretion system (T3SS) delivering effectors to trigger intestinal inflammation
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