A 65-year-old man is evaluated in the emergency department with a 2-day history of nonbloody watery diarrhea. He has more than ten bowel movements daily, with worsening abdominal pain and fever. The patient was hospitalized with cellulitis and received antibiotics 3 weeks ago. He was discharged with a 2-week course of oral antibiotics, which he completed 1 week ago.

　On physical examination, temperature is 38.6℃, blood pressure is 100／55 mmHg, pulse rate is 110／min, and respiration rate is 22／min. The abdomen is distended and tender to palpation.

　Laboratory studies indicate a leukocyte count of 27,000／μL. Blood, stool, and urine samples are obtained for culture, but the results are not back yet.