A 24-year-old man is brought to the emergency room with symptoms of acute intestinal obstruction. His temperature is 38degC (101degF), respirations are 25 per minute, and blood pressure is 120/80 mm Hg. Physical examination reveals a mass in the right lower abdominal quadrant. At laparoscopy, there are numerous small bowel strictures and a fistula extending into a loop of small bowel. Which of the following is the most likely diagnosis?