A 32-year-old man from Delhi presents with a 6-month history of bloody diarrhoea, abdominal pain, and weight loss. Clinical examination reveals pallor and mild abdominal tenderness. Laboratory investigations show haemoglobin 9.2 g/dL, albumin 2.8 g/dL, and elevated inflammatory markers (CRP 8.5 mg/dL, ESR 52 mm/h). Which investigation is most appropriate to confirm the diagnosis of inflammatory bowel disease and determine disease extent?