A 37-year-old man with a history of ulcerative colitis treated with infliximab and azathioprine is admitted to the hospital with abdominal distention, pain, and a fever of 98.6°F (37°C). After 24 hours of intravenous (IV) methylprednisolone, IV levofloxacin and metronidazole, bowel rest, and resuscitation, his heart rate has increased to 120 beats/min and his temperature is 102.2°F (39°C). Urgent computed tomography reveals megacolon without perforation. The best next step in management is: