Metabolic bone: X-linked hypophosphataemic rickets: 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: X-linked hypophosphataemic rickets

Sample item

A boy has bow legs, short stature and rickets that fails to respond to standard doses of vitamin D. Serum phosphate is low, calcium is normal and PTH is normal. Which defect is most likely?

A1-alpha hydroxylase deficiencyBDeficient dietary vitamin DCPHEX gene mutation causing excess FGF23 and renal phosphate wastingDType I collagen mutation

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

The answer and the reasoning

C. PHEX gene mutation causing excess FGF23 and renal phosphate wasting

X-linked hypophosphataemic rickets is the commonest inherited rickets, due to a PHEX mutation raising FGF23 and causing renal phosphate loss. Treatment is oral phosphate with calcitriol, or burosumab, a monoclonal antibody against FGF23.

Option by option

A. 1-alpha hydroxylase deficiency
1-alpha hydroxylase deficiency gives low calcium and high PTH.
B. Deficient dietary vitamin D
Dietary deficiency would respond to vitamin D and have raised PTH.
C. PHEX gene mutation causing excess FGF23 and renal phosphate wasting (correct)
Correct. PHEX mutation with high FGF23 explains phosphate wasting.
D. Type I collagen mutation
Type I collagen mutation causes osteogenesis imperfecta.

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 Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is C, PHEX gene mutation causing excess FGF23 and renal phosphate wasting. X-linked hypophosphataemic rickets is the commonest inherited rickets, due to a PHEX mutation raising FGF23 and causing renal phosphate loss.

Why is "1-alpha hydroxylase deficiency" not the answer?

1-alpha hydroxylase deficiency gives low calcium and high PTH.

Why is "Deficient dietary vitamin D" not the answer?

Dietary deficiency would respond to vitamin D and have raised PTH.

Why is "Type I collagen mutation" not the answer?

Type I collagen mutation causes osteogenesis imperfecta.

What concept does this question test?

It tests Metabolic bone: X-linked hypophosphataemic rickets in Orthopaedics. Treatment is oral phosphate with calcitriol, or burosumab, a monoclonal antibody against FGF23.

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: X-linked hypophosphataemic rickets?

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.