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

**Core Concept**
Paget's disease of bone is a chronic, localized skeletal disorder characterized by an abnormal breakdown and regrowth of bone tissue, leading to deformities and an increased risk of fractures. The disease is characterized by excessive osteoclastic bone resorption followed by an abnormal osteoblastic bone formation process.

**Why the Correct Answer is Right**
The patient's symptoms, including hip and back pain, headaches, hearing loss, and tinnitus, are consistent with Paget's disease of bone. The elevated plasma alkaline phosphatase levels indicate an increased bone turnover. The sharply demarcated lucencies in the skull x-ray and thickening of the pelvic brim on hip x-rays are classic radiographic findings of Paget's disease. The disease is often asymptomatic in its early stages, but as the disease progresses, it can lead to deformities and an increased risk of fractures.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because osteomalacia is characterized by softening of the bones due to impaired mineralization, which is not consistent with the patient's x-ray findings.

**Option B:** This option is incorrect because fibrous dysplasia is a benign bone disorder characterized by the replacement of normal bone with fibrous tissue, which does not account for the patient's elevated alkaline phosphatase levels.

**Option C:** This option is incorrect because osteoporosis is a condition of bone loss and decreased bone density, which is not consistent with the patient's x-ray findings of sharply demarcated lucencies.

**Clinical Pearl / High-Yield Fact**
Paget's disease of bone can be a marker for an underlying occult malignancy, such as prostate cancer or breast cancer, particularly in older adults.

**Correct Answer:** C.

✓ Correct Answer: B. Paget's disease