A 43-year-old man presents with severe diarrhea and hematochezia of insidious onset. Height is 6 ft 1 in and weight is 166 lb (BMI 21.9). Body temperature is 97.8 °F. There is no significant medical history apart from a "sensitive stomach," but his father has gastrointestinal motility issues, and his maternal aunt has multiple sclerosis (MS). Stool culture is negative. Endoscopic features include friability of the mucosa with mucosal erosions and areas of spontaneous bleeding. Colitis extends from the sigmoid to the splenic flexure.