An elderly male presents with headache, recurrent infections and multiple punched out lytic lesions of x-ray skull. Which of the following investigation will best help in establishing the diagnosis?

Patients presenting with symptoms such as headache, recurrent infections and multiple punched out lytic lesion on the skull suggest a diagnosis of Multiple Myeloma. It can be diagnosed by performing protein electrophoresis which shows a characteristic monoclonal spike visible in the beta or gamma globulin region. Ref: Current Medical Diagnosis and Treatment, 2012, Chapter 13