Active form of Vitamin D is:March 2011, March 2013 (a, e, g)
**Core Concept:** Vitamin D is a fat-soluble vitamin that plays a crucial role in calcium homeostasis, bone health, and immune system modulation. The active form of vitamin D is derived from its conversion in the liver and kidneys.
**Why the Correct Answer is Right:** The active form of vitamin D is calcitriol (1,25-dihydroxyvitamin D3). Calcitriol is produced in the kidneys through a series of enzymatic reactions involving vitamin D precursors, primarily pre-vitamin D3 and 25-hydroxyvitamin D3. These conversions are catalyzed by enzymes such as 25-hydroxyvitamin D-1Ξ±-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24A1).
**Why Each Wrong Option is Incorrect:**
A. Calcidiol (25-hydroxyvitamin D3) is the primary storage form of vitamin D in the body and is not the active form.
B. Calcipotriol (5Ξ±-dihydroxyvitamin D3) is an artificial analogue of vitamin D3 used as a topical agent in the treatment of psoriasis, not the active form of vitamin D.
C. Calcitriol (1,25-dihydroxyvitamin D3) is the active form of vitamin D, but the question specifically asks for the active form produced in the kidneys.
D. Calcidiol (25-hydroxyvitamin D3) is the primary storage form of vitamin D in the body and is not the active form.
**Clinical Pearl:** Monitoring vitamin D levels as 25-hydroxyvitamin D3 (25OHD3) or 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is essential in assessing vitamin D status and guiding treatment decisions, particularly in patients with chronic kidney disease or other conditions affecting vitamin D synthesis and activation.
**Correct Answer:** C. Calcitriol (1,25-dihydroxyvitamin D3) is the active form of vitamin D produced in the kidneys. Calcitriol plays a vital role in calcium and phosphorus homeostasis, bone remodeling, and immune system modulation.