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': 'Staghorn calculus is made out of: March 2004 \nSelect and explain. \nA. Oxalate, B. Phosphate, C. Uric acid, D. Cysteine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. B i.e. Phosphate', 'role': 'assistant'}]