Metabolic bone disease: vitamin D-dependent rickets type I: 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 I

Sample item

A 10-month-old has rickets with low calcium, raised PTH and a normal 25-hydroxyvitamin D, while calcitriol is low. Which defect is present and how is it treated?

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

The answer and the reasoning

A. 1-alpha-hydroxylase deficiency, treated with calcitriol

In vitamin D-dependent rickets type I the kidney cannot convert 25-hydroxyvitamin D to calcitriol because of 1-alpha-hydroxylase deficiency. The 25-hydroxyvitamin D is normal, calcitriol is low, and treatment is physiological doses of calcitriol.

Option by option

  • A. 1-alpha-hydroxylase deficiency, treated with calcitriol (correct)

    Correct. Normal 25-hydroxyvitamin D with low calcitriol indicates 1-alpha-hydroxylase deficiency, treated with calcitriol.

  • B. PHEX defect, treated with phosphate alone

    A PHEX defect gives hypophosphataemia with normal calcium and PTH.

  • C. Vitamin D receptor defect, treated with calcitriol

    A receptor defect would show high calcitriol and does not respond to calcitriol at physiological doses.

  • D. Fanconi syndrome, treated with sunlight exposure

    Fanconi syndrome causes phosphate wasting and acidosis, and sunlight would not correct it.

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 A, 1-alpha-hydroxylase deficiency, treated with calcitriol. In vitamin D-dependent rickets type I the kidney cannot convert 25-hydroxyvitamin D to calcitriol because of 1-alpha-hydroxylase deficiency.

Why is "PHEX defect, treated with phosphate alone" not the answer?

A PHEX defect gives hypophosphataemia with normal calcium and PTH.

Why is "Vitamin D receptor defect, treated with calcitriol" not the answer?

A receptor defect would show high calcitriol and does not respond to calcitriol at physiological doses.

Why is "Fanconi syndrome, treated with sunlight exposure" not the answer?

Fanconi syndrome causes phosphate wasting and acidosis, and sunlight would not correct it.

What concept does this question test?

It tests Metabolic bone disease: vitamin D-dependent rickets type I in Orthopaedics. The 25-hydroxyvitamin D is normal, calcitriol is low, and treatment is physiological doses of 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 I?

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.