This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal Radiology, Subject name: Radiology
A previously healthy 61 - year - old woman presents with a 3 - week history of altered bowel habit and rectal bleeding. She refuses to undergo colonoscopy. Which of the following is next best investigation? 
Select and explain. 
A. CT colonography, B. CT abdomen and pelvis, C. Barium enema, D. Barium meal follow through
A: CT colonography
Answer: A: CT colonography 
Explanation: CT colonography , CT abdomen and pelvis and barium enema are useful in this situation. But CT colonography is the most accurate of the three, enabling detection of small polyps. The conventional CT will only detect large tumours. The barium enema will only detect large polyps and frank tumours.