Endocrine drugs: nitisinone in tyrosinaemia: practice question

A Pharmacology 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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Pharmacology · Endocrine drugs: nitisinone in tyrosinaemia

Sample item

An infant with liver failure and a coagulopathy is found to have raised succinylacetone in urine, diagnosing hereditary tyrosinaemia type 1. Nitisinone is started. Which enzyme does it inhibit?

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

The answer and the reasoning

B. 4-hydroxyphenylpyruvate dioxygenase

In tyrosinaemia type 1 the defective enzyme is fumarylacetoacetate hydrolase, which allows toxic succinylacetone to accumulate. Nitisinone blocks the upstream enzyme 4-hydroxyphenylpyruvate dioxygenase, so these toxic metabolites are not formed. It is combined with a diet low in tyrosine and phenylalanine.

Option by option

  • A. Homogentisate 1,2-dioxygenase

    Homogentisate dioxygenase is deficient in alkaptonuria.

  • B. 4-hydroxyphenylpyruvate dioxygenase (correct)

    Correct. Nitisinone inhibits 4-hydroxyphenylpyruvate dioxygenase upstream of the defect.

  • C. Phenylalanine hydroxylase

    Phenylalanine hydroxylase is deficient in phenylketonuria.

  • D. Fumarylacetoacetate hydrolase

    Fumarylacetoacetate hydrolase is the deficient enzyme, not the drug target.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is B, 4-hydroxyphenylpyruvate dioxygenase. In tyrosinaemia type 1 the defective enzyme is fumarylacetoacetate hydrolase, which allows toxic succinylacetone to accumulate.

Why is "Homogentisate 1,2-dioxygenase" not the answer?

Homogentisate dioxygenase is deficient in alkaptonuria.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase is deficient in phenylketonuria.

Why is "Fumarylacetoacetate hydrolase" not the answer?

Fumarylacetoacetate hydrolase is the deficient enzyme, not the drug target.

What concept does this question test?

It tests Endocrine drugs: nitisinone in tyrosinaemia in Pharmacology. Nitisinone blocks the upstream enzyme 4-hydroxyphenylpyruvate dioxygenase, so these toxic metabolites are not formed. It is combined with a diet low in tyrosine and phenylalanine.

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 Endocrine drugs: nitisinone in tyrosinaemia?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.