**Core Concept**
Cherubism is a rare genetic disorder characterized by the abnormal growth of bone tissue, leading to the expansion of the jaw and facial bones. This condition is associated with an increase in bone density and a decrease in bone resorption.

**Why the Correct Answer is Right**
Cherubism is caused by mutations in the SH3BP2 gene, which regulates the activity of osteoclasts, the cells responsible for bone resorption. In Cherubism, the abnormal osteoclasts have reduced activity, leading to a decrease in bone resorption and an increase in bone density. As a result, serum calcium levels remain normal, and alkaline phosphatase levels may be elevated due to the increased bone turnover. The increased bone density and altered bone turnover lead to an increase in the serum alkaline phosphatase levels but normal serum calcium levels.

**Why Each Wrong Option is Incorrect**
**Option B:** In hypothyroidism, serum calcium levels are typically normal, but alkaline phosphatase levels are often elevated due to increased bone turnover. However, Cherubism is characterized by a specific pattern of bone growth and altered bone turnover, distinct from hypothyroidism.

**Option C:** Hyperparathyroidism is associated with elevated serum calcium levels, which is not the case in Cherubism. Hyperparathyroidism also leads to increased bone resorption, whereas Cherubism is characterized by reduced bone resorption.

**Option D:** Paget's disease is a condition characterized by abnormal bone growth and increased bone turnover, leading to elevated serum alkaline phosphatase levels. However, Paget's disease is not typically associated with the specific pattern of bone growth and altered bone turnover seen in Cherubism.

**Clinical Pearl / High-Yield Fact**
Cherubism is a rare genetic disorder that can be distinguished from other bone disorders based on its specific pattern of bone growth and altered bone turnover. It is essential to consider Cherubism in the differential diagnosis of patients with jaw and facial bone abnormalities.

✓ Correct Answer: A. Cherubism