A 34-year-old woman is admitted with fever and dull abdominal pain for 1 month. She has a history of pelvic tuberculosis. Physical examination shows abdominal wall rigidity with mild tenderness and rebound tenderness, most pronounced in the right lower quadrant. Barium meal X-ray reveals a 'stierlin sign' in the ileocecal region and loss of the normal angle between the ileum and cecum. The most likely diagnosis is: