This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ohopaedics Q Bank, Subject name: Orthopaedics
A patient presents with pain in back. Lab investigation shows elevated ESR. X-ray skull shows multiple punched out lytic lesions. Most impoant investigation to be done is: 
Select and explain. 
A. Serum acid phosphatase, B. CT head with contrast, C. Whole body scan, D. Serum electrophoresis
D: Serum electrophoresis
Answer: D: Serum electrophoresis 
Explanation: Multiple Myeloma. Multiple punched out lytic lesions of skull Dx- M bands on serum electrophoresis. Elevated ESR is an impoant finding in multiple myeloma Punched out lesions of skull are seen in Eosinophilic granuloma-bevelled edges (double contour) Multiple myeloma.