Metabolic bone disease: vitamin D-dependent rickets type II: practice question

A Orthopaedics 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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Orthopaedics · Metabolic bone disease: vitamin D-dependent rickets type II

Sample item

An infant with total alopecia has rickets that does not respond to usual doses of vitamin D. Serum calcium and phosphate are low, PTH is high, and calcitriol levels are markedly raised. What is the defect?

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

The answer and the reasoning

B. Mutation of the vitamin D receptor

Hereditary vitamin D-resistant rickets type II is caused by a defective vitamin D receptor, so target tissues cannot respond even though calcitriol is high. Alopecia is a marker of severe receptor defects. Type I, in contrast, is a deficiency of 1-alpha-hydroxylase with low calcitriol.

Option by option

  • A. Deficiency of renal 1-alpha-hydroxylase

    1-alpha-hydroxylase deficiency leads to low, not raised, calcitriol.

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

    Correct. A receptor defect causes rickets with high calcitriol and often alopecia.

  • C. Dietary deficiency of vitamin D

    Nutritional deficiency would respond to vitamin D and has low 25-hydroxyvitamin D.

  • D. Mutation of the PHEX gene

    PHEX mutation causes X-linked hypophosphataemic rickets with normal PTH and normal calcium.

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 Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is B, Mutation of the vitamin D receptor. Hereditary vitamin D-resistant rickets type II is caused by a defective vitamin D receptor, so target tissues cannot respond even though calcitriol is high.

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

1-alpha-hydroxylase deficiency leads to low, not raised, calcitriol.

Why is "Dietary deficiency of vitamin D" not the answer?

Nutritional deficiency would respond to vitamin D and has low 25-hydroxyvitamin D.

Why is "Mutation of the PHEX gene" not the answer?

PHEX mutation causes X-linked hypophosphataemic rickets with normal PTH and normal calcium.

What concept does this question test?

It tests Metabolic bone disease: vitamin D-dependent rickets type II in Orthopaedics. Alopecia is a marker of severe receptor defects. Type I, in contrast, is a deficiency of 1-alpha-hydroxylase with low 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 Metabolic bone disease: vitamin D-dependent rickets type II?

Pair each fracture with its classic complication and nerve injured. Then use the flashcards and the other Orthopaedics 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.