Amino acid metabolism: cystinosis: 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 · Amino acid metabolism: cystinosis

Sample item

A 14-month-old child has failure to thrive, polyuria, rickets and photophobia. Slit lamp examination shows corneal crystals, and white cells contain a high cystine content. Which defect is responsible?

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

The answer and the reasoning

C. A defective lysosomal cystine transporter, cystinosin

Cystinosis is a lysosomal storage disease in which the transporter cystinosin (CTNS) fails to export cystine from the lysosome, so crystals accumulate in cornea, kidney, marrow and other tissues. Renal Fanconi syndrome appears in infancy with polyuria, rickets and growth failure. Cysteamine depletes lysosomal cystine by forming a mixed disulfide that leaves through another transporter.

Option by option

  • A. A defective renal brush border dibasic amino acid transporter

    A dibasic transporter defect gives cystinuria with hexagonal urinary cystine stones, with no intracellular cystine storage.

  • B. Deficiency of sulfite oxidase

    Sulfite oxidase deficiency causes neurological damage and lens dislocation with raised sulfite.

  • C. A defective lysosomal cystine transporter, cystinosin (correct)

    Correct. Lysosomal cystine cannot leave the lysosome and builds up.

  • D. Deficiency of cystathionine beta-synthase

    Cystathionine beta-synthase deficiency causes homocystinuria with a low cystine level.

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 C, A defective lysosomal cystine transporter, cystinosin. Cystinosis is a lysosomal storage disease in which the transporter cystinosin (CTNS) fails to export cystine from the lysosome, so crystals accumulate in cornea, kidney, marrow and other tissues.

Why is "A defective renal brush border dibasic amino acid transporter" not the answer?

A dibasic transporter defect gives cystinuria with hexagonal urinary cystine stones, with no intracellular cystine storage.

Why is "Deficiency of sulfite oxidase" not the answer?

Sulfite oxidase deficiency causes neurological damage and lens dislocation with raised sulfite.

Why is "Deficiency of cystathionine beta-synthase" not the answer?

Cystathionine beta-synthase deficiency causes homocystinuria with a low cystine level.

What concept does this question test?

It tests Amino acid metabolism: cystinosis in Biochemistry. Renal Fanconi syndrome appears in infancy with polyuria, rickets and growth failure. Cysteamine depletes lysosomal cystine by forming a mixed disulfide that leaves through another transporter.

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 Amino acid metabolism: cystinosis?

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.