Active form of Vitamin -D –
**Core Concept**
The active form of Vitamin D plays a crucial role in calcium homeostasis, bone mineralization, and immune system modulation. It acts as a steroid hormone, exerting its effects by binding to specific receptors in target cells.
**Why the Correct Answer is Right**
The active form of Vitamin D, also known as calcitriol (1,25-dihydroxyvitamin D3), is synthesized in the kidneys from its precursor, calcidiol (25-hydroxyvitamin D3). This conversion is catalyzed by the enzyme 1-alpha-hydroxylase, which is regulated by parathyroid hormone (PTH) and calcium levels. Calcitriol then binds to the vitamin D receptor (VDR) in the nucleus of target cells, influencing gene expression and promoting calcium absorption in the intestine, bone resorption, and renal calcium reabsorption.
**Why Each Wrong Option is Incorrect**
**Option A:** Calcidiol (25-hydroxyvitamin D3) is the major circulating form of Vitamin D, but it is not the active form.
**Option B:** Ergocalciferol (Vitamin D2) is a precursor to calcidiol, but it is not the active form.
**Option C:** Cholecalciferol (Vitamin D3) is the form of Vitamin D found in animal products, but it is not the active form.
**Clinical Pearl / High-Yield Fact**
Vitamin D deficiency is associated with increased risk of osteoporosis, fractures, and autoimmune diseases, such as multiple sclerosis and type 1 diabetes. Patients with kidney or liver disease may require supplementation with calcitriol.
**Correct Answer:** C. Cholecalciferol is the precursor to calcidiol, not the active form of Vitamin D.