A 70-year-old African-American male who has been hospitalized for 2½ weeks for heart failure develops severe, persistent diarrhea. For the past 3 days he has had abdominal cramps and profuse, semi-formed stools without mucus or blood.
The patient’s current medications include captopril (Capoten), digoxin, furosemide (Lasix), subcutaneous heparin, spironolactone (Aldactone), and loperamide (Imodium). He has coronary artery disease, but has been relatively pain free since undergoing coronary artery bypass surgery 4 years ago. An appendectomy and cholecystectomy were performed in the past, and the patient has since been free of gastrointestinal disease.
On physical examination his blood pressure is 100/80 mm Hg, pulse 100 beats/min and regular, and temperature 37.0°C (98.6°F). He has mild jugular venous distention and crackles at both lung bases. Examination of his heart is unremarkable, although there is 1+ dependent edema. His abdomen is diffusely tender without masses or organomegaly. Findings on a rectal examination are normal.
The results of routine laboratory tests, including a CBC, chemistry profile, EKG, and urinalysis, are all normal. The stool examination shows numerous white blood cells.
Of the following, the most likely diagnosis is: