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
X-ray of a young man shows hetrotopic calcification around bilateral knee joints. Next investigation would be- 
Select and explain. 
A. Serum phosphate, B. Serum calcium, C. Serum PTH, D. Serum Alkaline phosphatase
D: Serum Alkaline phosphatase
Answer: D: Serum Alkaline phosphatase 
Explanation: None