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': 'Which of the following forms radiopaque stones: \nSelect and explain. \nA. Xanthine., B. Oxalate., C. Uric acid., D. None.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Radiopaque stones: cysteine, oxalate & struvite\nRadiolucent stones: xanthine, uric acid.\nAllopurinol may lead to xanthine stones.', 'role': 'assistant'}]