Q. A 72-year-old man comes to the geriatric clinic because of shortness of breath and back pain. The patient has a long history of back pain that has recently become intolerable. The patient's medical history is significant for hypertension and a remote history of IV drug abuse. He also reports recently applying for a hearing aid because of hearing loss. His blood pressure is 140/60 mm Hg, heart rate 100/min, and respiratory rate 18/min. Physical examination reveals bilateral lower extremity pitting edema. Chest auscultation reveals bilateral fine basal crepitations. Examination of his spine is significant for localized redness and tenderness over his lumbar vertebrae. An x-ray of his back reveals areas of mixed osteolytic and osteoblastic lesions. Electrolyte and kidney function are determined to be normal. Which of the following is the most likely cause of this patient's symptoms?