An elderly patient presents with a prolonged history of weakness and lethargy. On examination he is found to be anemic and stool is positive for occult blood. Which of the following is the investigation of choice?

Elderly patient presenting with history of weakness, lethargy, anemia and stool positive for occult blood is showing features of colorectal carcinoma. Colonoscopy is the diagnostic procedure of choice in patients with a clinical history suggestive of colon cancer or in patients showing abnormalities suggestive of cancer on radiographic imaging. Colonoscopy permits biopsy for pathologic confirmation of malignancy. When colonoscope is unable to reach the cecum or when a tumor is obstructing the passage of colonoscope, barium enema or CT colonography should be performed. Ref: Current medical diagnosis and treatment 2012, Chapter 39.