**Core Concept**
Paget's disease is a chronic focal skeletal disorder characterized by abnormal bone remodeling, leading to deformities and potential complications. It affects the bone structure by promoting excessive bone resorption and formation, resulting in weakened bones.

**Why the Correct Answer is Right**
The most serious complication of Paget's disease is the increased risk of osteosarcoma, a type of bone cancer. This occurs due to the abnormal bone remodeling process, which can lead to genetic mutations in the bone cells. The rapid bone turnover and abnormal cellular activity create an environment conducive to malignant transformation. The skull, pelvis, femur, and humerus are commonly affected bones.

**Why Each Wrong Option is Incorrect**
**Option A:** Bone fractures are a common complication of Paget's disease, but they are not the most serious complication. Fractures are often a result of the weakened bone structure, but they can be managed with proper treatment.
**Option B:** Deafness is a complication associated with Paget's disease, particularly when it affects the skull. However, it is not the most serious complication, and there are other options that pose a greater threat to the patient's life.
**Option C:** Neurological complications, such as epilepsy or seizures, can occur in patients with Paget's disease, especially when the disease affects the skull or spine. However, these complications are not as life-threatening as osteosarcoma.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that the risk of osteosarcoma increases with the duration and severity of Paget's disease. Patients with a history of Paget's disease should be closely monitored for signs and symptoms of bone cancer, such as pain, swelling, or a palpable mass.

**Correct Answer: D. Osteosarcoma.**