The kidney activates which vitamin to enable calcium absorption?

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

Answer: (A)

The key idea is how calcium gets absorbed from the gut and how vitamin activation drives that process. The kidney activates vitamin D into its active form, calcitriol. Vitamin D comes from diet or skin synthesis and is first converted in the liver to 25-hydroxyvitamin D. The kidney then converts that to 1,25-dihydroxyvitamin D (calcitriol) in response to parathyroid hormone when calcium is low. Calcitriol increases the intestinal absorption of calcium by upregulating calcium-binding proteins and transport mechanisms in enterocytes, so more calcium enters the bloodstream. Without this activation step, calcium absorption would be much less efficient, even with adequate calcium intake. The other vitamins listed have important roles (A for vision/epithelial health, C for collagen/wound healing, K for coagulation and some bone aspects) but they don’t directly enable intestinal calcium absorption like active vitamin D does.