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': 'All of the following are radio-opaque stones except: September 2009 \nSelect and explain. \nA. Calcium, B. Struvite, C. Uric acid, D. Cystine', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. C: Uric acid', 'role': 'assistant'}]