A 65-year-old man comes to the office with a 6-month history of back pain, fatigue, and recurrent infections. He has also noticed recent weight loss, nausea, and increased thirst. His medical history is significant for hypertension, hyperlipidemia, and a 20-pack-year smoking history. Physical examination shows tenderness over the thoracic spine and mild pallor. Laboratory testing shows a serum calcium level of 11.2 mg/dL (normal range 8.6-10.2 mg/dL), serum creatinine of 2.4 mg/dL (normal range 0.6-1.3 mg/dL), and hemoglobin of 9.2 g/dL (normal range 13.5-17.5 g/dL). Plain radiographs of the spine show multiple lytic lesions in the mid-thoracic vertebrae. Which of the following is the most likely diagnosis?