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

Crohn disease can affect the entire GI tract from mouth to anus. Right lower quadrant pain, tenderness, and an inflammatory mass would suggest involvement of the terminal ileum. As opposed to ulcerative colitis (a pure mucosal disease), Crohn disease, with full-thickness involvement of the gut wall, can lead to fistula and deep abscess formation. Skip lesions (ie, segmental involvement) also suggest Crohn disease; granuloma formation on biopsies would also support the diagnosis of Crohn disease.