Vitamins and minerals: vitamin D-dependent rickets type 1: 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: vitamin D-dependent rickets type 1

Sample item

A 9-month-old has rickets with hypocalcaemia, raised PTH and raised alkaline phosphatase. Serum 25-hydroxyvitamin D is normal, calcitriol is low and the child responds to small physiological doses of calcitriol. Which defect is most likely?

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

The answer and the reasoning

B. Renal 1-alpha-hydroxylase deficiency

Vitamin D-dependent rickets type 1 is an autosomal recessive CYP27B1 defect that stops conversion of 25-hydroxyvitamin D to calcitriol. Substrate is normal, calcitriol is low and physiological calcitriol cures it. In type 2, calcitriol is high because the receptor is defective.

Option by option

  • A. Hepatic 25-hydroxylase deficiency

    A 25-hydroxylase defect would give a low 25-hydroxyvitamin D.

  • B. Renal 1-alpha-hydroxylase deficiency (correct)

    Correct. Normal 25-hydroxyvitamin D with low calcitriol points to the renal activation step.

  • C. Vitamin D receptor mutation

    A receptor defect gives a high calcitriol level and resistance to treatment.

  • D. Excess FGF23 production

    FGF23 excess causes hypophosphataemic rickets with normal calcium and PTH.

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 B, Renal 1-alpha-hydroxylase deficiency. Vitamin D-dependent rickets type 1 is an autosomal recessive CYP27B1 defect that stops conversion of 25-hydroxyvitamin D to calcitriol.

Why is "Hepatic 25-hydroxylase deficiency" not the answer?

A 25-hydroxylase defect would give a low 25-hydroxyvitamin D.

Why is "Vitamin D receptor mutation" not the answer?

A receptor defect gives a high calcitriol level and resistance to treatment.

Why is "Excess FGF23 production" not the answer?

FGF23 excess causes hypophosphataemic rickets with normal calcium and PTH.

What concept does this question test?

It tests Vitamins and minerals: vitamin D-dependent rickets type 1 in Biochemistry. Substrate is normal, calcitriol is low and physiological calcitriol cures it. In type 2, calcitriol is high because the receptor is defective.

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 and minerals: vitamin D-dependent rickets type 1?

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.