Nephrology and electrolytes: cystinuria transport defect: practice question

A General Medicine 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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General Medicine · Nephrology and electrolytes: cystinuria transport defect

Sample item

A 20-year-old man has had recurrent renal stones since adolescence, and his brother is similarly affected. Urine microscopy shows flat hexagonal crystals and the sodium nitroprusside test is positive. Which defect is responsible?

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

The answer and the reasoning

B. Defective proximal tubular reabsorption of dibasic amino acids

In cystinuria an autosomal recessive defect in the proximal tubular transporter for cystine, ornithine, lysine and arginine causes cystine to be lost in urine. Cystine is poorly soluble, so hexagonal crystals and radio-opaque stones form. Management is high fluid intake, urine alkalinisation and, if needed, thiol drugs such as tiopronin.

Option by option

  • A. Defective proximal tubular reabsorption of filtered phosphate ions

    This describes hypophosphataemic rickets, which does not give hexagonal crystals.

  • B. Defective proximal tubular reabsorption of dibasic amino acids (correct)

    Correct. Loss of the dibasic amino acid transporter leaves insoluble cystine in the urine.

  • C. Defective intestinal and tubular transport of neutral amino acids

    This describes Hartnup disease, which causes pellagra-like rash and ataxia rather than stones.

  • D. Defective proximal tubular reabsorption of filtered glucose via SGLT2

    This describes renal glycosuria, which does not form stones.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is B, Defective proximal tubular reabsorption of dibasic amino acids. In cystinuria an autosomal recessive defect in the proximal tubular transporter for cystine, ornithine, lysine and arginine causes cystine to be lost in urine.

Why is "Defective proximal tubular reabsorption of filtered phosphate ions" not the answer?

This describes hypophosphataemic rickets, which does not give hexagonal crystals.

Why is "Defective intestinal and tubular transport of neutral amino acids" not the answer?

This describes Hartnup disease, which causes pellagra-like rash and ataxia rather than stones.

Why is "Defective proximal tubular reabsorption of filtered glucose via SGLT2" not the answer?

This describes renal glycosuria, which does not form stones.

What concept does this question test?

It tests Nephrology and electrolytes: cystinuria transport defect in General Medicine. Cystine is poorly soluble, so hexagonal crystals and radio-opaque stones form. Management is high fluid intake, urine alkalinisation and, if needed, thiol drugs such as tiopronin.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Nephrology and electrolytes: cystinuria transport defect?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.