A 26-year-old man presents with intermittent crampy abdominal pain, diarrhea without noticeable blood, and weight loss of 15 lb over 10 months. The bowel symptoms, including diarrhea, wake him from sleep. On a few occasions, he has had fevers, nausea, and vomiting. The patient is an architect, and he describes his work as being stressful; he resumed smoking cigarettes a year ago. On examination, the patient is a slender man with normal vital signs. He has an oral aphthous ulcer and poorly localized lower abdominal to midabdominal tenderness without peritoneal signs. Anal and rectal examinations are normal, and a stool guaiac test is negative. Stool leukocytes are present. The hematocrit is 34%. Results of examination with flexible sigmoidoscopy are normal. Which of the following is the most likely diagnosis for this patient?