What is Causing This Patient's Abdominal Tenderness and Aphthous Ulcers?

A 23-year-old woman presents with a recurrence of severe, crampy abdominal pain and a prolonged history of diarrhea. Her chart shows previous complaints of fatigue and joint pain, as well as evidence of weight loss during the past 2 years. Examination reveals aphthous oral ulcers and right lower quadrant abdominal tenderness. An endoscopy later demonstrates focal ulcerations adjacent to ...