This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, 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 protoscope, B. Barium enema, C. CT chest, D. MRI-abdomen and pelvis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rigid protoscope is of least significance in carcinoma rectum with fixed mass', 'role': 'assistant'}]