🦠 Clostridium difficile (C. diff): Diarrhea You Can’t Ignore on the USMLE
Clostridium difficile causes pseudomembranous colitis, a toxin-mediated colitis most often triggered by antibiotic use. It frequently appears in Step 1 and Step 2 clinical vignettes involving recent hospitalization, antibiotic exposure, and profuse diarrhea.
USMLE Tip: Diarrhea + recent clindamycin or fluoroquinolone = always suspect C. diff.
🧬 Pathophysiology
C. diff overgrows after normal gut flora is wiped out by antibiotics
Produces toxins A and B, which cause mucosal inflammation and necrosis
Leads to formation of pseudomembranes in the colon
🚨 High-Risk Antibiotics
| Drug Class | Examples |
|---|---|
| Penicillins | Amoxicillin, ampicillin |
| Cephalosporins | Ceftriaxone, cefepime |
| Clindamycin | Classic USMLE association |
| Fluoroquinolones | Ciprofloxacin, levofloxacin |
🧪 Clinical Presentation
| Symptom | Description |
|---|---|
| Diarrhea | Watery, foul-smelling, profuse |
| Abdominal pain | Cramping, lower abdominal discomfort |
| Fever | Usually present |
| Leukocytosis | Elevated white count |
| Severe cases | Toxic megacolon, perforation, sepsis |
🧫 Diagnostic Approach
| Test | Notes |
|---|---|
| Stool C. diff antigen + toxin PCR | Most sensitive and specific combination |
| Stool culture | Slow, less commonly used |
| Colonoscopy | Only for uncertain/severe cases; shows pseudomembranes |
💊 Treatment Strategy
| Severity | Treatment |
|---|---|
| Mild/Moderate | Oral vancomycin or fidaxomicin |
| Severe | High-dose oral vanco ± rectal vanco |
| Fulminant | Oral + rectal vanco + IV metronidazole |
| Recurrence | Tapered vanco, fidaxomicin, or fecal transplant |
❌ Avoid anti-motility agents (e.g., loperamide) → risk of toxic megacolon!
📚 Sample USMLE Vignette
A 72-year-old woman hospitalized for pneumonia develops watery diarrhea after 6 days of ceftriaxone. She is febrile with abdominal tenderness and WBC 22,000. What is the next best step?
✅ Answer: Test stool for C. diff toxin and start oral vancomycin
📲 Call to Action
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📲 Download the King of the Curve App for:
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Frequently Asked Questions (FAQs)
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