**Core Concept**
Calcitonin is a hormone secreted by the thyroid gland, playing a crucial role in calcium and phosphorus metabolism. It helps regulate the levels of these minerals in the blood by inhibiting osteoclast activity and promoting calcium deposition in bones.

**Why the Correct Answer is Right**
Calcitonin works by binding to its receptor on osteoclasts, leading to a decrease in their activity and subsequently reducing bone resorption. This hormone also promotes the deposition of calcium in bones by increasing the activity of osteoblasts. Additionally, calcitonin inhibits the release of calcium from bones by blocking the activity of osteoclasts. This ultimately helps maintain normal calcium levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as calcitonin's primary role is not in the regulation of phosphorus metabolism.
**Option B:** This option is incorrect as glucagon, not calcitonin, is involved in the mobilization of calcium from bones.
**Option C:** This option is incorrect as calcitonin does not directly stimulate the release of parathyroid hormone (PTH).

**Clinical Pearl / High-Yield Fact**
It's essential to remember that calcitonin is often used in the diagnosis of medullary thyroid carcinoma, as elevated levels of calcitonin can be a marker for this condition.

**Correct Answer: D. Regulation of calcium and phosphorus metabolism.