**Question:** Mechanism of Calcitriol is:

A. Increases calcium absorption in the intestine
B. Inhibits parathyroid hormone (PTH) secretion
C. Promotes bone resorption
D. Inhibits osteoclastic activity

**Correct Answer: B. Inhibits parathyroid hormone (PTH) secretion**

**Core Concept:** Calcitriol is the active form of vitamin D, which plays a vital role in calcium and phosphate homeostasis. It interacts with the parathyroid glands, bones, and intestines.

**Why the Correct Answer is Right:** Vitamin D, in its active form (calcitriol), primarily acts on parathyroid glands to maintain calcium and phosphate homeostasis in the body. Calcitriol suppresses parathyroid hormone (PTH) secretion, which in turn reduces calcium release from bones and enhances calcium absorption in the intestines. This mechanism helps regulate blood calcium levels and prevent hypercalcemia and hypocalcemia.

**Why Each Wrong Option is Incorrect:**

A. Calcitriol does increase calcium absorption in the intestines, but its primary function is not solely related to this action.
B. Calcitriol does not directly stimulate bone resorption. Instead, it inhibits PTH secretion, which drives bone resorption.
C. Calcitriol's main action is not promoting osteoclastic activity; rather, it suppresses PTH secretion.

**Clinical Pearl:** Understanding the role of calcitriol in calcium and phosphate homeostasis is crucial for managing conditions like hypoparathyroidism, hyperparathyroidism, and vitamin D deficiency. Adequate vitamin D status ensures optimal bone health, neuromuscular function, and cardiovascular system functioning.