In human physiology, which lipid-soluble vitamin functions as a hormone by regulating gene transcription through nuclear receptors?

• A. Vitamin K
• B. Vitamin E
• C. Vitamin A
• D. Vitamin D

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the classification of vitamins based on their solubility. Vitamins can be water-soluble or lipid-soluble. Lipid-soluble vitamins include Vitamins A, D, E, and K.
2. Step 2: Recognize that some lipid-soluble vitamins act as hormones by entering cells and binding to nuclear receptors, which then regulate gene transcription.
3. Step 3: Identify the specific vitamin that functions as a hormone by binding to nuclear receptors to regulate gene expression. This vitamin is known for its role in calcium homeostasis and bone metabolism.
4. Step 4: Compare the functions of the given vitamins: Vitamin A is involved in vision and gene regulation but primarily through retinoic acid receptors; Vitamin E acts mainly as an antioxidant; Vitamin K is important for blood clotting.
5. Step 5: Conclude that Vitamin D is the lipid-soluble vitamin that functions as a hormone by regulating gene transcription through nuclear receptors, influencing various physiological processes.