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