A 50-year-old patient complains of pain in the bones and ribs. Complete blood count: erythrocytes - 3.31012/L, hemoglobin - 100 g/L, leukocytes - 6.5109/L, segmented neutrophils - 50%, lymphocytes - 32%, monocytes - 18%, ESR - 62 mm/hour. Skull X-ray shows multiple small regular-shaped defects. Sternal punctate contains 30% of plasma cells. What is the most likely diagnosis in this case?