Vitamins and minerals: X-linked hypophosphataemic rickets: 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: X-linked hypophosphataemic rickets

Sample item

A 3-year-old boy has bowed legs and short stature. Rickets persists despite usual doses of vitamin D. Serum calcium and PTH are normal, phosphate is low and alkaline phosphatase is high; his mother is short with mild genu varum. What is the mechanism of the low phosphate?

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

The answer and the reasoning

A. Excess FGF23 activity causing phosphate wasting in the proximal tubule

X-linked dominant hypophosphataemic rickets results from inactivating PHEX mutations, which raise FGF23. FGF23 inhibits renal phosphate reabsorption and suppresses calcitriol synthesis, so calcium and PTH stay normal while phosphate is low. Treatment is oral phosphate with calcitriol, or burosumab, a monoclonal antibody against FGF23.

Option by option

  • A. Excess FGF23 activity causing phosphate wasting in the proximal tubule (correct)

    Correct. Raised FGF23 causes renal phosphate loss with normal calcium and PTH.

  • B. Mutant vitamin D receptor causing end-organ resistance to circulating calcitriol

    Vitamin D receptor defects give hypocalcaemia and raised PTH, often with alopecia.

  • C. Absent renal 1-alpha-hydroxylase activity reducing calcitriol synthesis in the kidney

    Absent 1-alpha-hydroxylase gives low calcium and raised PTH, unlike this child.

  • D. Secondary hyperparathyroidism from long-standing dietary calcium deficiency here

    Calcium deficiency raises PTH, but PTH is normal here.

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, Excess FGF23 activity causing phosphate wasting in the proximal tubule. X-linked dominant hypophosphataemic rickets results from inactivating PHEX mutations, which raise FGF23.

Why is "Mutant vitamin D receptor causing end-organ resistance to circulating…" not the answer?

Vitamin D receptor defects give hypocalcaemia and raised PTH, often with alopecia.

Why is "Absent renal 1-alpha-hydroxylase activity reducing calcitriol synthes…" not the answer?

Absent 1-alpha-hydroxylase gives low calcium and raised PTH, unlike this child.

Why is "Secondary hyperparathyroidism from long-standing dietary calcium defi…" not the answer?

Calcium deficiency raises PTH, but PTH is normal here.

What concept does this question test?

It tests Vitamins and minerals: X-linked hypophosphataemic rickets in Biochemistry. FGF23 inhibits renal phosphate reabsorption and suppresses calcitriol synthesis, so calcium and PTH stay normal while phosphate is low. 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 Vitamins and minerals: X-linked hypophosphataemic rickets?

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.