A 28-year-old man with Crohn's disease (ileocolonic) on azathioprine monotherapy for 2 years presents with a 4-week history of progressive abdominal pain, non-bloody diarrhea (5–6 stools/day), and weight loss (3 kg). Examination reveals a palpable mass in the right lower abdomen. CT enterography shows a 4 cm segment of ileac stricture with proximal bowel dilatation, no fistulae, and no free perforation. CRP is 5.2 mg/dL (mildly elevated). What is the most appropriate next step in management?