A 55 year old man had abdominal cramps and diarrhea since the past month. he has a history of similar episodes since the past 20 yrs. Each episode lasts about 2 weeks and resolves without treatment. Stool sample was positive for occult blood. laboratory studies did not show any ova or parasites in the stool. colonoscopy was performed and it showed diffuse and uninterrupted mucosal inflammation and superficial ulceration extending from rectum to ascending colon. He is at greatest risk for developing which of the following complications?