Which organ provides the last step in producing vitamin D?

• A. Kidneys
• B. Liver
• C. Skin
• D. Lungs

Answer: (A)

Vitamin D becomes active through a three-step process. The skin first produces vitamin D3 when exposed to sunlight, giving the initial precursor. That precursor is carried to the liver, where it’s hydroxylated to 25-hydroxyvitamin D (calcidiol). The final activation happens in the kidneys, where 25-hydroxyvitamin D is further hydroxylated by 1-alpha-hydroxylase to form 1,25-dihydroxyvitamin D (calcitriol), the biologically active hormone. This renal step is regulated to keep calcium and phosphate in balance, so the kidneys are where the last step of producing active vitamin D occurs. The liver handles the prior step, and the skin provides the starting material; the lungs are not involved in this pathway.