A 32-year-old man from Mumbai with a 3-year history of Crohn disease (ileocolic involvement) on azathioprine and mesalamine presents with a 2-week history of fever (38.5°C), right lower abdominal pain with a palpable mass, and mild diarrhea. Abdominal ultrasound shows a 4 cm × 3 cm hypoechoic mass in the right iliac fossa with surrounding bowel wall thickening. Laboratory findings: Hb 9.2 g/dL, WBC 12,500/μL, CRP 8.5 mg/dL. What is the most likely diagnosis?