This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urology, Subject name: Anatomy
[{'content': 'Best investigation for ureteric stones is: \nSelect and explain. \nA. CT, B. CECT, C. USG, D. IVP', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Helical CT without oral or intravenous contrast is the preferred imaging modality for patients with renal colic or suspected urolithiasis" -- Smith\'s Urology Ref : Smith\'s Urology 17/e p81, 261', 'role': 'assistant'}]