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
A patient with pain in back. Lab investigation shows elevated ESR. X-ray skull shows multiple punched out lytic lesions. Most imp. 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: This patient has : -

Multiple punched-out lytic lesions of the skull.
Elevated ESR
Back pain
Diagnosis is multiple myeloma.
Multiple myelomas is diagnosed by M bands on serum electrophoresis.