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: Orthopaedics
[{'content': 'X-ray of a young man shows hetrotopic calcification around bilateral knee joints. Next investigation would be- \nSelect and explain. \nA. Serum phosphate, B. Serum calcium, C. Serum PTH, D. Serum Alkaline phosphatase', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]