A 90-year-old male complains of hip and back pain. He has also developed headaches, hearing loss, and tinnitus. On physical exam the skull appears enlarged, with prominent superficial veins. There is marked kyphosis, and the bones of the leg appear deformed. Plasma alkaline phosphatase is elevated. A skull x-ray shows sharply demarcated lucencies in the frontal, parietal, and occipital bones. X-rays of the hip show thickening of the pelvic brim. The most likely diagnosis is