A 28-year-old woman with a 3-year history of Crohn's disease (involving terminal ileum and proximal colon) on azathioprine presents with acute onset of severe abdominal pain, fever (38.5°C), and a palpable right lower quadrant mass. Laboratory tests show WBC 14,500/μL, CRP 12 mg/dL, and albumin 2.9 g/dL. Which investigation is most appropriate to diagnose the suspected complication and guide management?