Which organ is primarily responsible for producing the activated form of vitamin D?

• A. Skin
• B. Kidney
• C. Liver
• D. Intestine

Answer: (B)

The activated form of vitamin D is produced primarily in the kidney. In the kidney, the enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D (the liver-derived circulating form) into 1,25-dihydroxyvitamin D (calcitriol), the hormonally active form. This activation step is essential because calcitriol increases intestinal absorption of calcium and phosphate, supports bone mobilization when needed, and helps with renal calcium reabsorption. While sunlight exposure on the skin generates the initial vitamin D precursor and the liver provides a storage form, the kidney is where the final activation occurs. The intestine responds to calcitriol by increasing absorption, but it does not activate vitamin D itself.