A 45-year-old woman undergoes an uneventful laparoscopic cholecystectomy for which she receives 1 dose of cephalosporin. One week later, she returns to the emergency room with fever, nausea, and copious diarrhea and is suspected of having pseudomembranous colitis. She is afebrile and has no peritoneal signs on abdominal examination. She has a mild leukocytosis with a left shift. Which of the following is the appropriate initial management strategy?