28-year-old recently divorced man with no significant past medical history presents to the emergency room with progressive lower abdominal pain and cramping over the past 4 days, both of which are relieved by defecation. He has suffered from substantial bloody and mucoid diarrhea during this time. His temperature is 102.8°F. Laboratory studies reveal an elevated white blood cell (WBC) count and a high erythrocyte sedimentation rate. Sigmoidoscopy reveals extensive rectal and sigmoid hyperemia and edema, numerous superficial ulcerations, and small focal mucosal hemorrhages, many of which have suppurative centers. Significant intestinal narrowing is seen in the distal transverse colon. These findings most likely suggest a diagnosis of which of the following?