A 62-year-old male presents with dizziness and fatigue for 6 months, and irregular fever for 3 months. Physical exam: anemic appearance, multiple bean-sized lymph nodes in the neck and supraclavicular region, liver 2 cm below the costal margin, spleen 4 cm below the costal margin. Laboratory tests: Hb 65g/L, red blood cells (RBC) showing rouleaux formation, positive Bence Jones protein in urine, erythrocyte sedimentation rate (ESR) 152mm/h, immunoglobulin G (IgG) 12g/L, immunoglobulin A (IgA) 3g/L, immunoglobulin M (IgM) 11g/L, bone marrow smear shows lymphoplasmacytic cells accounting for 30%. The most likely diagnosis is: