## **Core Concept**
Calcitonin is a hormone involved in calcium and phosphorus metabolism. It is secreted by a specific type of cell in the thyroid gland. Understanding the source of calcitonin secretion is crucial for endocrinology and metabolism.

## **Why the Correct Answer is Right**
The correct answer, **C. Para-follicular cells (C cells)**, is right because calcitonin is secreted by the para-follicular cells, also known as C cells, of the thyroid gland. These cells are distinct from the follicular cells that produce thyroid hormones (T3 and T4). When calcium levels in the blood rise, calcitonin is released to help lower them by promoting the deposition of calcium into bone and inhibiting osteoclast activity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the Chief cells, primarily found in the stomach, secrete pepsinogen, which is involved in protein digestion, not calcitonin.
- **Option B:** This option is incorrect as Acinar cells are related to the pancreas and are involved in the production of digestive enzymes, not calcitonin.
- **Option D:** This option is incorrect because Follicular cells of the thyroid gland produce and secrete thyroid hormones (T3 and T4), not calcitonin.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that calcitonin levels can be elevated in medullary thyroid carcinoma, a type of thyroid cancer that originates from the para-follicular C cells. This fact is clinically relevant for the diagnosis and management of this condition.

## **Correct Answer:** C. Para-follicular cells (C cells).