An 18-year-old man presents with abdominal pain, malnutrition, and diarrhea. He reports that he has had a bowel movement every time he eats, with some bleeding through the rectum, for the past 6 weeks. Pink bismuth and loperamide have not diminished the symptoms. On examination, he has mild tenderness in the lower left abdominal quadrant. Stool cultures for pathogens are negative. He is 5' 10" tall and weighs 180 lb (BMI, 25.8). His temperature is 99.4 °F, blood pressure 125/78 mm Hg, respiratory rate 22 breaths/min, and pulse 65 beats/min. Laboratory evaluation reveals a C-reactive protein level of 36 mg/L. On colonoscopy with histology, there is inflammation and erosions of the mucosa involving three quarters of the length of the large bowel. The patient is perinuclear anti-neutrophil cytoplasmic antibody (pANCA)–positive. He reports that he is in good health otherwise except for recently diagnosed uveitis, and he is usually active in outdoor sports.