Calcium homeostasis: vitamin D: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Calcium homeostasis: vitamin D

Sample item

A 45-year-old man with chronic kidney disease has low serum calcium and a raised PTH. Which step in vitamin D activation is mainly impaired?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. 1-alpha-hydroxylation of 25-hydroxycholecalciferol in the proximal tubule

Cholecalciferol is hydroxylated in the liver to 25-hydroxycholecalciferol, then in the proximal renal tubule by 1-alpha-hydroxylase to the active calcitriol. In chronic kidney disease this enzyme activity is lost, so calcitriol falls, intestinal calcium absorption declines and secondary hyperparathyroidism develops. Phosphate retention aggravates the problem.

Option by option

  • A. Intestinal absorption of dietary cholecalciferol

    Intestinal absorption of vitamin D is not the defect; the activation of it is.

  • B. 25-hydroxylation of cholecalciferol in the liver

    Hepatic 25-hydroxylation is usually preserved until severe liver disease.

  • C. 1-alpha-hydroxylation of 25-hydroxycholecalciferol in the proximal tubule (correct)

    Correct. The renal activation step is lost in kidney disease.

  • D. Photolysis of 7-dehydrocholesterol in the skin

    Skin synthesis depends on sunlight exposure and is not affected by kidney disease.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, 1-alpha-hydroxylation of 25-hydroxycholecalciferol in the proximal tubule. Cholecalciferol is hydroxylated in the liver to 25-hydroxycholecalciferol, then in the proximal renal tubule by 1-alpha-hydroxylase to the active calcitriol.

Why is "Intestinal absorption of dietary cholecalciferol" not the answer?

Intestinal absorption of vitamin D is not the defect; the activation of it is.

Why is "25-hydroxylation of cholecalciferol in the liver" not the answer?

Hepatic 25-hydroxylation is usually preserved until severe liver disease.

Why is "Photolysis of 7-dehydrocholesterol in the skin" not the answer?

Skin synthesis depends on sunlight exposure and is not affected by kidney disease.

What concept does this question test?

It tests Calcium homeostasis: vitamin D in Physiology. In chronic kidney disease this enzyme activity is lost, so calcitriol falls, intestinal calcium absorption declines and secondary hyperparathyroidism develops. Phosphate retention aggravates the problem.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Calcium homeostasis: vitamin D?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.