For the patient in the preceding question, after obtaining diagnostic samples, the most appropriate management would include which of the following?

c. Oral vancomycin

The efficacy of vancomycin in treatment of PMC has been so well documented that it must be considered the most reliable treatment for the disease. No isolates of C difficile are resistant to vancomycin. Metronidazole, bacitracin, and teicoplanin are alternatives when vancomycin is not tolerated. C difficile isolates are almost always susceptible to vancomycin at concentrations of lower than 5 mg/l, and few isolates have been identified that require more than 16 mg/l. Because vancomycin is poorly absorbed into the systemic circulation when given orally at a dosage of 125 to 500 mg four times per day, concentrations far exceeding 16 mg/l, the maximum concentration required for inhibition of C difficile, are easily achieved in stools. Even patients with profuse diarrhea achieve adequate concentrations of vancomycin in stools with these regimens. While small amounts of vancomycin may be absorbed systemically, toxic serum concentrations have never been reported with oral therapy, even when the patient has renal failure.