A 38-year-old male with a 2-year history of confirmed cirrhosis presents with anemia and gingival bleeding. Physical exam shows chronic liver disease facies, spider angiomas on the chest, and splenomegaly (5 cm below the costal margin). Laboratory tests: Hb 80g/L, WBC 4×10<sup>9</sup>/L, PLT 53×10<sup>9</sup>/L, IgG 22g/L, IgA 15g/L, IgM 1.0g/L. Bone marrow smear shows 10% mature plasma cells. The most appropriate diagnosis is: