This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'A 67 year old man presented initially after a single episode of gross painless haematuria. An excretory urogram demonstrated a 1.5 mm round filing defect in the right lower renal infundibulum.The best study to obtain next is - \nSelect and explain. \nA. Urinary cytology, B. Cystoscopy, C. Ultrasonography, D. Retrograde pyelography', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]