Image of the Week: What's causing this recurrent abdominal pain?

An otherwise healthy 15-year-old male presents to his family physician with a 6-month history of progressive bloating, recurrent abdominal pain, bloody diarrhea, and tenesmus. His abdomen is tender to palpation in the right lower quadrant, and rectal examination reveals perianal skin tags and occult blood in his stool. A diagnosis of ulcerative colitis is suspected and a colonoscopy is booked. ...