Calcitriol is the active form of which vitamin?

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

Answer: (C)

The main idea is knowing which vitamin has its active metabolite as calcitriol. Calcitriol is the active form of vitamin D. It’s produced when vitamin D, obtained from skin synthesis or diet, is first converted in the liver to calcidiol and then in the kidney to calcitriol (1,25-dihydroxyvitamin D3) by the enzyme 1-alpha-hydroxylase. Calcitriol acts as a hormone by binding the vitamin D receptor in target tissues, especially the gut and bones, and increases intestinal absorption of calcium and phosphate, supporting calcium homeostasis and bone mineralization. It works alongside parathyroid hormone to regulate calcium levels. The other vitamins listed do not form this active metabolite.