Question

The functiolly active form of Vitamin D is:

• A. Cholecalciferol
• B. Ergocalciferol
• C. Dehydrocholesterol
• D. Calcitriol

Hint:

Cholecalciferol ($D_3$) = Ictive precursor.
Calcidiol = Main storage/circulating form ($25\text{-OH } D_3$).
Calcitriol = Active hormol form ($1,25\text{-(OH)}_2 D_3$).

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the chemically fully active hormol metabolite derived from Vitamin D in the human body.

Step 2: Key Formula or Approach:
Vitamin D undergoes a series of two sequential hydroxylation reactions in the liver and kidneys to reach its biologically active form: \[ \text{7-Dehydrocholesterol} \xrightarrow{\text{UV Light}} \text{Cholecalciferol (Vit } D_3) \xrightarrow[\text{Liver}]{25\text{-hydroxylase}} 25(\text{OH})D_3 \xrightarrow[\text{Kidney}]{1\alpha\text{-hydroxylase}} 1,25(\text{OH})_2D_3 \]

Step 3: Detailed Explation:

• Cholecalciferol (Vitamin D$_3$): Produced in skin exposed to UV-B radiation or ingested via diet; it is biologically ictive precursor.

• 25-Hydroxycholecalciferol (Calcidiol): Major circulating precursor form produced in the liver by 25-hydroxylase.

• 1,25-Dihydroxycholecalciferol (Calcitriol): Formed in rel proximal tubules via $1\alpha$-hydroxylase. Calcitriol is the fully functiol active steroid-like hormone.

• Biological Action: Calcitriol binds nuclear Vitamin D Receptors (VDR) to regulate gene expression responsible for intestil calcium and phosphate absorption and bone mineralization.

Step 4: Fil Answer:
The active hormone form of Vitamin D is calcitriol ($1,25\text{-dihydroxycholecalciferol}$).