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
Investigation of choice for diagnosis of CA colon 
Select and explain. 
A. Colonoscopy, B. Double contrast barium enema, C. Triple phase CT, D. Viual colonoscopy
A: Colonoscopy
Answer: A: Colonoscopy 
Explanation: Colonoscopy is the "gold standard" for establishing the diagnosis of colon cancer. It permits biopsy of a tumour to verify the diagnosis while allowing inspection of the entire colon to exclude metachronous polyps or cancers; the incidence of a synchronous cancer is approximately 3%. Colonoscopy is generally performed even after a cancer is detected by barium enema to obtain a biopsy specimen and to detect (and remove) small polyps that may be missed by the contrast studyRef: Sabiston, 20e, page: 1398