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 of - \nSelect and explain. \nA. Oxalate, B. Phosphate, C. Uric acid, D. Cystine', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Phosphate", 'role': 'assistant'}]