Vitamin D and Calcium Absorption — MFDS Part 1 MCQ

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ModeratePhysiologyVitamin D and Calcium AbsorptionMFDS Part 1

In response to reduced plasma calcium, endocrine adaptation increases active transcellular calcium absorption across the intestinal epithelium. Which hormone exerts the principal direct effect on enterocytes?

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Correct answer: C — 1,25-dihydroxycholecalciferol (calcitriol)

The correct answer is C. Calcitriol, or 1,25-dihydroxycholecalciferol, is the principal direct hormonal regulator of intestinal calcium absorption. It binds the nuclear vitamin D receptor in enterocytes and alters expression of proteins involved in transcellular calcium entry, intracellular handling and basolateral extrusion, including calbindin-related pathways. Parathyroid hormone is the most plausible distractor: it raises calcium partly by stimulating renal formation of calcitriol, but it is not the principal hormone acting directly on enterocytes. Calcitonin primarily opposes rises in plasma calcium through effects on bone and kidney rather than driving intestinal active calcium transport. Thyroxine has no primary physiological role in this pathway, while glucocorticoids such as cortisol tend to reduce calcium absorption rather than facilitate it.

Reference: electronic Medicines Compendium, Calcitriol 0.5 microgram Capsules, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties, updated 23 February 2026. https://www.medicines.org.uk/emc/product/101903/smpc