A 68-year-old male presents with progressive anemia for 1 year, severe flank pain for 3 months, and recent rib fracture due to sneezing. Liver and spleen are non-palpable. Hemoglobin (Hb) 90g/L, white blood cell (WBC) count 3.4×10<sup>9</sup>/L, platelets (PLT) 70×10<sup>9</sup>/L. Erythrocyte sedimentation rate (ESR) 120 mm/h, urinary protein (+++), serum protein electrophoresis (SPEP) shows monoclonal protein (M protein), elevated immunoglobulin G (IgG), and 60% abnormal plasma cells in bone marrow. Lumbar spine X-ray reveals osteoporosis and round punched-out lytic lesions. The most likely diagnosis is: