Amino acid metabolism: tyrosinaemia type I: 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: tyrosinaemia type I

Sample item

A 5-month-old infant has failure to thrive, hepatomegaly, a bleeding tendency with prolonged prothrombin time, rickets and a cabbage-like odour. Urine contains succinylacetone. Which drug is used to prevent further toxic metabolite formation?

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

The answer and the reasoning

C. Nitisinone, an inhibitor of 4-hydroxyphenylpyruvate dioxygenase

Tyrosinaemia type I is due to deficiency of fumarylacetoacetate hydrolase, the last enzyme of tyrosine degradation. Fumarylacetoacetate and its derivative succinylacetone accumulate, damaging liver and renal tubules and predisposing to hepatocellular carcinoma. Nitisinone blocks the upstream enzyme 4-hydroxyphenylpyruvate dioxygenase so these metabolites are not formed, and a diet low in tyrosine and phenylalanine is added.

Option by option

  • A. Cysteamine, which depletes lysosomal cystine

    Cysteamine is used in cystinosis to lower lysosomal cystine.

  • B. Sodium benzoate, a nitrogen scavenger

    Sodium benzoate is a nitrogen scavenger for urea cycle defects and does not alter tyrosine catabolism.

  • C. Nitisinone, an inhibitor of 4-hydroxyphenylpyruvate dioxygenase (correct)

    Correct. Blocking the upstream step prevents formation of fumarylacetoacetate and succinylacetone.

  • D. Sapropterin, a synthetic tetrahydrobiopterin

    Sapropterin is a cofactor therapy for tetrahydrobiopterin-responsive phenylketonuria.

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, Nitisinone, an inhibitor of 4-hydroxyphenylpyruvate dioxygenase. Tyrosinaemia type I is due to deficiency of fumarylacetoacetate hydrolase, the last enzyme of tyrosine degradation.

Why is "Cysteamine, which depletes lysosomal cystine" not the answer?

Cysteamine is used in cystinosis to lower lysosomal cystine.

Why is "Sodium benzoate, a nitrogen scavenger" not the answer?

Sodium benzoate is a nitrogen scavenger for urea cycle defects and does not alter tyrosine catabolism.

Why is "Sapropterin, a synthetic tetrahydrobiopterin" not the answer?

Sapropterin is a cofactor therapy for tetrahydrobiopterin-responsive phenylketonuria.

What concept does this question test?

It tests Amino acid metabolism: tyrosinaemia type I in Biochemistry. Fumarylacetoacetate and its derivative succinylacetone accumulate, damaging liver and renal tubules and predisposing to hepatocellular carcinoma. Nitisinone blocks the upstream enzyme 4-hydroxyphenylpyruvate dioxygenase so these metabolites are not formed, and a diet low in tyrosine and phenylalanine is added.

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 metabolism: tyrosinaemia type I?

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.