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Answer
The answer is A) A positive stool culture growing C. difficile alone is sufficient to establish the diagnosis.
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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
| Option | Verdict | Explanation |
|---|---|---|
| B) Diarrhea ≥3 unformed stools/24h for ≥2 days, no other recognized cause | ✅ Valid criterion | This is the required clinical component |
| C) Detection of toxins A or B in stool | ✅ Valid criterion | Direct toxin detection is a core laboratory criterion |
| D) Detection of pseudomembranes in the colon | ✅ Valid criterion | Endoscopic 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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