Which of the following is an action of vitamin D?

• A. Intestinal absorption of calcium
• B. Secretion of parathyroid hormone
• C. Inhibition of calcium absorption
• D. Increased calcitonin release

Answer: (A)

Vitamin D, in its active form calcitriol, increases calcium uptake from the gut. It does this by promoting the production of calcium-transport proteins in intestinal cells—like calbindin and the channels that bring calcium into the cells—so more calcium is absorbed from the dietary lumen into the bloodstream. This gut absorption is a key part of how the body maintains normal serum calcium levels.

This is why the action described is the best choice: it directly explains how vitamin D affects calcium by boosting intestinal absorption. The other options don’t fit because PTH secretion is driven by calcium sensing and is not a direct action of vitamin D, while vitamin D does not inhibit calcium absorption or increase calcitonin release.