A 28-year-old woman presents with 18 hours of periumbilical pain that has migrated to the right lower quadrant, accompanied by anorexia and low-grade fever (38.2°C). On examination, she has rebound tenderness at McBurney's point and a positive Rovsing's sign. Her WBC is 12,500/μL with left shift. What is the investigation of choice to confirm the diagnosis?