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
A punch biopsy shows carcinoma rectum with fixed mass. X-ray chest normal, which of the following is least useful investigation- 
Select and explain. 
A. Rigid proctoscope, B. Barium enema, C. CT chest, D. MRI- abdomen and pelvis
A: Rigid proctoscope
Answer: A: Rigid proctoscope 
Explanation: None