A 52-year-old male presents with back pain for 6 months and gingival bleeding for 2 weeks. Physical exam: moderate anemic appearance, liver palpable 1cm below the costal margin, spleen just palpable below the costal margin. Laboratory tests: Hb 80g/L, WBC 3.8×10<sup>9</sup>/L, PLT 56×10<sup>9</sup>/L. Urinary protein (+++), ESR 105mm/h, skeletal X-ray shows wedge compression of L<sub>1</sub> and L<sub>3</sub>, and moth-eaten destruction of multiple ribs. Serum calcium 3.25 mmol/L, IgG 5.0g/L, IgA 2.1g/L, IgM 0.5g/L, IgD 0.003g/L, urine Bence Jones protein (-). What is the most likely diagnosis?