A 55-year-old male presents with renal failure. He gives a history of mild bone pains for the last 7 years. X-ray pelvis shows osteolytic lesions. Serum electrophoresis reveals a M spike. Peripheral blood evaluation showed rouleaux formation of RBCs with 35% plasma cells. Bone marrow examination showed a increase in plasma cells with aberrant antigen expression. The most likely diagnosis is: