A 55-year-old man complains of pain in his back, fatigue and occasional confusion. He admits to polyuria and polydipsia. An X-ray examination reveals numerous lytic lesions in the lumbar vertebral bodies. Laboratory studies disclose hypoalbuminemia, mild anemia, and thrombocytopenia. A monoclonal Igk peak is demonstrated by serum electrophoresis. Urinalysis shows 4+ proteinuria. A bone marrow biopsy discloses foci of plasma cells, which account for 18% of all hematopoietic cells. What is the appropriate diagnosis?