Amino acid metabolism: tyrosinaemia type II: 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 II

Sample item

A 4-year-old boy has painful, photophobic eye lesions with pseudodendritic corneal erosions, painful thickening of the palms and soles, and mild intellectual disability. Plasma tyrosine is markedly raised, but succinylacetone is absent in urine. Which enzyme is deficient?

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

The answer and the reasoning

D. Tyrosine aminotransferase

Deficiency of cytosolic hepatic tyrosine aminotransferase causes tyrosinaemia type II (Richner-Hanhart syndrome). High tyrosine crystallises in tissues, producing corneal lesions and palmoplantar keratosis. Absence of succinylacetone excludes type I, which is caused by the hydrolase defect.

Option by option

  • A. Homogentisate 1,2-dioxygenase

    Homogentisate dioxygenase deficiency gives alkaptonuria with dark urine and ochronosis, not high tyrosine.

  • B. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency gives raised phenylalanine, with tyrosine low or normal.

  • C. Fumarylacetoacetate hydrolase

    Deficiency of the hydrolase gives type I disease with liver failure, rickets and urinary succinylacetone.

  • D. Tyrosine aminotransferase (correct)

    Correct. This transamination step is the first in tyrosine catabolism; its failure gives eye and skin lesions.

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 D, Tyrosine aminotransferase. Deficiency of cytosolic hepatic tyrosine aminotransferase causes tyrosinaemia type II (Richner-Hanhart syndrome).

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

Homogentisate dioxygenase deficiency gives alkaptonuria with dark urine and ochronosis, not high tyrosine.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency gives raised phenylalanine, with tyrosine low or normal.

Why is "Fumarylacetoacetate hydrolase" not the answer?

Deficiency of the hydrolase gives type I disease with liver failure, rickets and urinary succinylacetone.

What concept does this question test?

It tests Amino acid metabolism: tyrosinaemia type II in Biochemistry. High tyrosine crystallises in tissues, producing corneal lesions and palmoplantar keratosis. Absence of succinylacetone excludes type I, which is caused by the hydrolase defect.

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: tyrosinaemia type II?

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.