A 28-year-old man from Delhi presents with a 6-month history of abdominal pain, diarrhea, and weight loss. Colonoscopy reveals patchy areas of inflammation in the terminal ileum and sigmoid colon, with normal intervening mucosa. Biopsy shows full-thickness inflammation with non-caseating granulomas. The pattern shown corresponds to the condition marked **A** in the diagram. Which of the following complications is MOST characteristic of this entity?