A 58-year-old woman with a history of colorectal cancer (treated 3 years ago) presents with progressive abdominal distension, constipation, and colicky pain for 3 days. On examination, the abdomen is distended with visible peristalsis. Abdominal X-ray shows a dilated colon with a transition point at the sigmoid region. What is the most appropriate next investigation to confirm the diagnosis and assess resectability?