A 72-year-old man is found to have a normocytic anaemia with Hb 95 g/L (MCV 90 fL). His ferritin, B12, and folate levels are normal. Renal function is normal. A blood film shows rouleaux formation. Serum protein electrophoresis reveals an M-protein band (IgG kappa) of 35 g/L. Bone marrow biopsy confirms plasma cell infiltration of 30%. X-rays show lytic lesions in the skull. What is the most likely diagnosis?