Vitamins: vitamin D resistance: 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: vitamin D resistance

Sample item

An 18-month-old child has rickets that does not respond to usual doses of vitamin D, sparse hair with alopecia totalis, hypocalcaemia, hypophosphataemia, high alkaline phosphatase and high PTH. Serum 1,25-dihydroxyvitamin D is markedly raised. Which defect is most likely?

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

The answer and the reasoning

A. Mutation of the vitamin D receptor

Hereditary vitamin D-resistant rickets type II is caused by vitamin D receptor mutations. Target tissues cannot respond to calcitriol, so levels rise markedly, and alopecia is characteristic. In 1-alpha-hydroxylase deficiency calcitriol is low and the child responds to calcitriol.

Option by option

  • A. Mutation of the vitamin D receptor (correct)

    End-organ resistance explains rickets with very high calcitriol and alopecia.

  • B. Deficiency of renal 1-alpha-hydroxylase

    A 1-alpha-hydroxylase defect gives low, not high, calcitriol and responds to physiological calcitriol replacement.

  • C. Excess FGF23 activity from PHEX mutation

    X-linked hypophosphataemia from FGF23 excess gives low phosphate with normal calcium and normal PTH, and no alopecia.

  • D. Deficiency of hepatic 25-hydroxylase

    A 25-hydroxylase defect gives low 25-hydroxyvitamin D and low calcitriol and responds to vitamin D.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Mutation of the vitamin D receptor. Hereditary vitamin D-resistant rickets type II is caused by vitamin D receptor mutations.

Why is "Deficiency of renal 1-alpha-hydroxylase" not the answer?

A 1-alpha-hydroxylase defect gives low, not high, calcitriol and responds to physiological calcitriol replacement.

Why is "Excess FGF23 activity from PHEX mutation" not the answer?

X-linked hypophosphataemia from FGF23 excess gives low phosphate with normal calcium and normal PTH, and no alopecia.

Why is "Deficiency of hepatic 25-hydroxylase" not the answer?

A 25-hydroxylase defect gives low 25-hydroxyvitamin D and low calcitriol and responds to vitamin D.

What concept does this question test?

It tests Vitamins: vitamin D resistance in Biochemistry. Target tissues cannot respond to calcitriol, so levels rise markedly, and alopecia is characteristic. In 1-alpha-hydroxylase deficiency calcitriol is low and the child responds to calcitriol.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: vitamin D resistance?

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.