Amino acid transport: 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.

Try it

Biochemistry · Amino acid transport: cystinosis

Sample item

An 8-month-old infant has polyuria, polydipsia, failure to thrive and rickets with a normal anion gap acidosis, glycosuria with normal blood glucose, phosphaturia and aminoaciduria. Slit-lamp examination shows corneal crystals. Which therapy acts by depleting the intracellular compound that accumulates?

APenicillamineBCysteamineCSapropterinDNitisinone

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

The answer and the reasoning

B. Cysteamine

Cystinosis is caused by a defect of cystinosin, the lysosomal cystine exporter, so cystine crystallises in lysosomes of kidney, cornea and other tissues, producing renal Fanconi syndrome. Cysteamine enters lysosomes and forms a mixed disulfide with cysteine that leaves through the lysine transporter, lowering cystine content. Renal transplantation is needed in later renal failure.

Option by option

A. Penicillamine
Penicillamine chelates copper in Wilson disease and also binds cystine in cystinuria, but not lysosomal cystine in cystinosis.
B. Cysteamine (correct)
Correct. Cysteamine forms a cysteine-cysteamine disulfide that exits the lysosome and so reduces cystine.
C. Sapropterin
Sapropterin supplies tetrahydrobiopterin in BH4-responsive phenylketonuria.
D. Nitisinone
Nitisinone blocks 4-hydroxyphenylpyruvate dioxygenase in tyrosinaemia type I.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Cysteamine. Cystinosis is caused by a defect of cystinosin, the lysosomal cystine exporter, so cystine crystallises in lysosomes of kidney, cornea and other tissues, producing renal Fanconi syndrome.

Why is "Penicillamine" not the answer?

Penicillamine chelates copper in Wilson disease and also binds cystine in cystinuria, but not lysosomal cystine in cystinosis.

Why is "Sapropterin" not the answer?

Sapropterin supplies tetrahydrobiopterin in BH4-responsive phenylketonuria.

Why is "Nitisinone" not the answer?

Nitisinone blocks 4-hydroxyphenylpyruvate dioxygenase in tyrosinaemia type I.

What concept does this question test?

It tests Amino acid transport: cystinosis in Biochemistry. Cysteamine enters lysosomes and forms a mixed disulfide with cysteine that leaves through the lysine transporter, lowering cystine content. Renal transplantation is needed in later renal failure.

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 Amino acid transport: 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.