This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'A punch biopsy shows carcinoma rectum with fixed mass. X-ray chest normal, which of the following is least useful investigation- \nSelect and explain. \nA. Rigid proctoscope, B. Barium enema, C. CT chest, D. MRI- abdomen and pelvis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]