Nutrition and malnutrition: hypophosphataemic rickets: practice question

A Pediatrics 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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Pediatrics · Nutrition and malnutrition: hypophosphataemic rickets

Sample item

A toddler with bowed legs and wrist widening has low phosphate, normal calcium, normal PTH and a raised ALP. He does not respond to routine vitamin D. What is the defect?

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

The answer and the reasoning

C. PHEX mutation with excess FGF23 causing renal phosphate loss

X-linked hypophosphataemic rickets results from a PHEX mutation that raises FGF23, causing renal phosphate wasting and inadequate 1,25-dihydroxyvitamin D. Calcium and PTH stay normal, unlike nutritional rickets. Treatment is oral phosphate with calcitriol.

Option by option

  • A. Impaired distal tubular hydrogen ion secretion

    Distal renal tubular acidosis gives acidosis and hypokalaemia.

  • B. Vitamin D receptor mutation

    Vitamin D receptor mutation causes low calcium with a high PTH and alopecia.

  • C. PHEX mutation with excess FGF23 causing renal phosphate loss (correct)

    Correct. High FGF23 wastes phosphate with normal calcium and PTH.

  • D. Deficient 1-alpha hydroxylase activity

    1-alpha hydroxylase deficiency causes low calcium and a high 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 Pediatrics question

What is the answer to this Pediatrics question?

The best answer is C, PHEX mutation with excess FGF23 causing renal phosphate loss. X-linked hypophosphataemic rickets results from a PHEX mutation that raises FGF23, causing renal phosphate wasting and inadequate 1,25-dihydroxyvitamin D.

Why is "Impaired distal tubular hydrogen ion secretion" not the answer?

Distal renal tubular acidosis gives acidosis and hypokalaemia.

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

Vitamin D receptor mutation causes low calcium with a high PTH and alopecia.

Why is "Deficient 1-alpha hydroxylase activity" not the answer?

1-alpha hydroxylase deficiency causes low calcium and a high PTH.

What concept does this question test?

It tests Nutrition and malnutrition: hypophosphataemic rickets in Pediatrics. Calcium and PTH stay normal, unlike nutritional rickets. Treatment is oral phosphate with 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 Nutrition and malnutrition: hypophosphataemic rickets?

Memorise milestones and the vaccine schedule as tables and revise them often. Then use the flashcards and the other Pediatrics 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.