Active form of vitamin D is
**Core Concept**
Vitamin D plays a crucial role in maintaining calcium homeostasis and bone health. It undergoes two hydroxylation reactions to become its active form, which is essential for intestinal calcium absorption and bone mineralization.
**Why the Correct Answer is Right**
The correct answer is 1,25-dihydroxyvitamin D (calcitriol), which is the active form of vitamin D. This process involves two hydroxylation reactions: first in the liver, where 25-hydroxyvitamin D is formed, and then in the kidneys, where 1,25-dihydroxyvitamin D is synthesized through the action of the enzyme 1-alpha-hydroxylase. This active form binds to the vitamin D receptor (VDR) in the nucleus, regulating gene expression involved in calcium homeostasis.
**Why Each Wrong Option is Incorrect**
* **Option A:** 25-hydroxyvitamin D is an intermediate form of vitamin D, not the active form.
* **Option B:** Ergocalciferol (D2) is a precursor to 25-hydroxyvitamin D, not the active form.
* **Option D:** None of the above options are correct.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the active form of vitamin D, calcitriol, is synthesized in the kidneys, and its production can be impaired in patients with chronic kidney disease, leading to secondary hyperparathyroidism.
**Correct Answer: C. 1,25-dihydroxyvitamin D (calcitriol)**