In human physiology, which lipid-soluble vitamin can function as a hormone by being converted to calcitriol, regulating calcium and phosphate homeostasis?

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

Answer: (A)

Verified step by step guidance

1. Understand that lipid-soluble vitamins are vitamins that dissolve in fats and oils and can be stored in the body's fatty tissue.
2. Recall that among the lipid-soluble vitamins (A, D, E, and K), only one is converted into an active hormone form that regulates calcium and phosphate balance in the body.
3. Identify that Vitamin D is converted in the body to calcitriol, which acts as a hormone to regulate calcium and phosphate homeostasis, essential for bone health.
4. Recognize that Vitamin E primarily functions as an antioxidant, Vitamin K is involved in blood clotting, and Vitamin A is important for vision and cellular growth, but none of these are converted into calcitriol.
5. Conclude that the lipid-soluble vitamin functioning as a hormone by being converted to calcitriol is Vitamin D.