A 35-year-old white man presents with diarrhea, weight loss, and right lower quadrant (RLQ) pain. On examination, a tender mass is noted in the RLQ; the fecal occult blood test is positive. Colonoscopy shows segmental areas of inflammation. Barium small bowel series shows nodular thickening of the terminal ileum.Select the most likely cause of lower gastrointestinal bleeding