A 69-year-old woman is brought to the emergency room after falling off a step stool and fracturing her hip. Her past medical history is significant for several bouts of pneumonia during the past year. Laboratory results indicate a normal white blood cell count, but platelets are decreased, and the erythrocyte sedimentation rate (ESR) is elevated. X-rays reveal multiple lytic bone lesions. Serum electrophoresis demonstrates an M-protein spike. Which of the following is the MOST likely diagnosis?