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The answer is confirmed. Here is the full explanation:

Answer: A) A positive stool culture growing C. difficile alone is sufficient to establish the diagnosis

This is the EXCEPT answer - it is NOT part of the diagnostic criteria.

Why Option A is Wrong (and is the Answer)

According to Harrison's Principles of Internal Medicine (2025), the diagnosis of C. difficile infection (CDI) requires a combination of criteria - a positive culture alone is insufficient:
"The diagnosis of CDI is based on a combination of clinical criteria: (1) diarrhea (≥3 unformed stools per 24 h for ≥2 days) with no other recognized cause PLUS (2) detection of toxin A or B in the stool, detection of toxin-producing C. difficile by NAAT/PCR or culture, OR visualization of pseudomembranes in the colon."
  • Harrison's Principles of Internal Medicine 22E, Ch. 139
The key point: a positive stool culture alone does NOT establish the diagnosis because:
  • C. difficile can colonize the gut asymptomatically (colonization ≠ infection)
  • Even patients colonized with C. difficile who develop diarrhea may have another cause
  • Stool culture detects organism presence but not active toxin production; toxin confirmation is needed alongside clinical symptoms

Why the Other Options ARE Valid Diagnostic Criteria

OptionVerdictExplanation
B) Diarrhea ≥3 unformed stools/24h for ≥2 days, no other recognized cause✅ Valid criterionThis is the required clinical component
C) Detection of toxins A or B in stool✅ Valid criterionDirect toxin detection is a core laboratory criterion
D) Detection of pseudomembranes in the colon✅ Valid criterionEndoscopic visualization of pseudomembranes (pseudomembranous colitis) confirms CDI

Clinical Context (the Scenario)

The colonoscopy image shows pseudomembranous colitis - yellowish-white plaques on an inflamed, hemorrhagic colonic mucosa. This is the classic endoscopic appearance of severe CDI in an ICU patient on broad-spectrum antibiotics and a PPI (both are known risk factors). The pseudomembranes you see are diagnostic criterion D in action.
The answer is A - a positive culture alone cannot diagnose CDI; you always need toxin evidence or pseudomembranes, coupled with compatible clinical symptoms.
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