A 30-year-old man of Ashkenazi Jewish descent presents with abdominal pain, malnutrition, and diarrhea. He reports that he has four to five bowel movements per day with some bleeding through his rectum. Probiotics and loperamide have not alleviated his symptoms. On examination, he has mild tenderness in the lower left abdominal quadrant. Stool culture results for common pathogens are negative. He is 5 ft 8 in tall and weighs 160 lb (body mass index, 24.3). He has a temperature of 99.2 °F, blood pressure of 125/78 mm Hg, respiratory rate of 22 breaths/min, and pulse of 65 beats/min. Laboratory results reveal a C-reactive protein level of 20 mg/L. On colonoscopy with histologic analysis, there is inflammation and erosion of the mucosa of about half the length of the large bowel. His test result is positive for perinuclear antineutrophil cytoplasmic antibodies (pANCA). The patient reports that he is in good health otherwise except for ankylosing spondylitis.