Antimicrobials USMLE Step 1 Practice Question
A 42-year-old man with community-acquired pneumonia is started on azithromycin. After 3 days, he develops severe watery diarrhea with abdominal cramping and fever (38.9°C). Vital signs show HR 102/min, BP 128/76 mmHg, RR 18/min. Laboratory testing reveals WBC 13,800/μL and positive C. difficile toxin B in stool. Abdominal examination shows mild tenderness without peritoneal signs. Which mechanism explains this adverse effect of azithromycin?
Answer choices
- AHypersensitivity reaction to azithromycin metabolites
- BDisruption of normal colonic flora allowing C. difficile overgrowthCorrect answer
- CDirect mucosal irritation by the macrolide
- DInhibition of intestinal motility leading to bacterial stasis
- EDirect toxin production by azithromycin in the colon
- FMacrolide-induced increased intestinal permeability leading to bacterial translocation
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