A 62-year-old man from Delhi presents with persistent back pain and fatigue for 3 months. Serum calcium is 11.2 mg/dL, creatinine 2.1 mg/dL, and hemoglobin 8.5 g/dL. Serum protein electrophoresis shows an M-spike of 3.5 g/dL. Bone marrow biopsy reveals 45% plasma cells. Skeletal survey shows lytic lesions in the skull and pelvis. What is the most appropriate next step in management?