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: Surgery
[{'content': 'What is this study? \nSelect and explain. \nA. Retrograde Pyelogram, B. Ultrasound, C. X Ray KUB, D. Contrast CT KUB', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Retrograde ureterogram demonstrating the collecting system. The radiolucent filling defect in the renal pelvis is caused by radiolucent calculus', 'role': 'assistant'}]