Metronidazole requires anaerobic activation. A patient with Clostridioides difficile colitis fails metronidazole therapy. The most likely pharmacodynamic explanation specific to the colonic environment is:

• A Metronidazole is metabolized by the human gut epithelial CYP3A4 before reaching the lumen
• B In severe C. difficile colitis, mucosal inflammation reduces intraluminal metronidazole concentrations; oral vancomycin achieves direct luminal bactericidal activity irrespective of mucosal integrity ✓
• C C. difficile has developed metronidazole resistance via rdxA gene mutation, identical to H. pylori resistance
• D Metronidazole's colonic bioactivation requires Bacteroides fragilis as a cofactor, absent in C. difficile infection

Explanation

Oral metronidazole for CDI is absorbed in the upper GI tract; it reaches the colon via systemic circulation and transudation through the inflamed mucosa. In severe or fulminant CDI, mucosal perfusion and transudation are impaired, resulting in subtherapeutic intraluminal metronidazole concentrations. Oral vancomycin, in contrast, is virtually unabsorbed from the GI tract and achieves direct high intraluminal concentrations regardless of mucosal integrity — hence its superiority in severe CDI (IDSA guidelines now recommend vancomycin or fidaxomicin over metronidazole for all disease severities). True metronidazole resistance in C. difficile is rare and does not clinically drive therapeutic failure.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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