Vitamins and minerals: measuring vitamin D status: practice question

A Biochemistry 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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Biochemistry · Vitamins and minerals: measuring vitamin D status

Sample item

A clinic wants a single blood test to judge a patient's vitamin D stores. Which measurement is most appropriate, and why?

A7-dehydrocholesterol, the skin precursor that falls when stores are lowB1,25-dihydroxyvitamin D, which directly mirrors total body vitamin D storesC24,25-dihydroxyvitamin D, the principal active hormone found in plasmaD25-hydroxyvitamin D, the main circulating form with a long half-life

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

The answer and the reasoning

D. 25-hydroxyvitamin D, the main circulating form with a long half-life

Vitamin D is hydroxylated in the liver to 25-hydroxyvitamin D, which circulates at nanomolar levels with a half-life of weeks and reflects both dietary and cutaneous supply. Calcitriol has a half-life of hours, is tightly regulated by PTH, calcium and phosphate, and can be normal or even high in deficiency.

Option by option

A. 7-dehydrocholesterol, the skin precursor that falls when stores are low
7-dehydrocholesterol is a skin precursor that is not measured to assess stores.
B. 1,25-dihydroxyvitamin D, which directly mirrors total body vitamin D stores
Calcitriol is regulated by PTH and may be normal or raised in deficiency.
C. 24,25-dihydroxyvitamin D, the principal active hormone found in plasma
24,25-dihydroxyvitamin D is an inactivation product and not the active hormone.
D. 25-hydroxyvitamin D, the main circulating form with a long half-life (correct)
Correct. 25-hydroxyvitamin D is stable and tracks supply, so it is the standard test of status.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, 25-hydroxyvitamin D, the main circulating form with a long half-life. Vitamin D is hydroxylated in the liver to 25-hydroxyvitamin D, which circulates at nanomolar levels with a half-life of weeks and reflects both dietary and cutaneous supply.

Why is "7-dehydrocholesterol, the skin precursor that falls when stores are l…" not the answer?

7-dehydrocholesterol is a skin precursor that is not measured to assess stores.

Why is "1,25-dihydroxyvitamin D, which directly mirrors total body vitamin D…" not the answer?

Calcitriol is regulated by PTH and may be normal or raised in deficiency.

Why is "24,25-dihydroxyvitamin D, the principal active hormone found in plasma" not the answer?

24,25-dihydroxyvitamin D is an inactivation product and not the active hormone.

What concept does this question test?

It tests Vitamins and minerals: measuring vitamin D status in Biochemistry. Calcitriol has a half-life of hours, is tightly regulated by PTH, calcium and phosphate, and can be normal or even high in deficiency.

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 Vitamins and minerals: measuring vitamin D status?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.