Amino acid metabolism: tyrosine: 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: tyrosine

Sample item

A 45-year-old man has arthritis of the spine and knees, bluish-black discolouration of the ear cartilage and urine that turns dark when left standing. Which enzyme is deficient?

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

The answer and the reasoning

C. Homogentisate 1,2-dioxygenase

Alkaptonuria is an autosomal recessive deficiency of homogentisate oxidase (dioxygenase). Homogentisic acid accumulates, oxidises and polymerises to a pigment that deposits in cartilage (ochronosis) and causes early degenerative arthritis. Urine darkens on standing or with alkali.

Option by option

  • A. Tyrosinase

    Tyrosinase deficiency causes oculocutaneous albinism.

  • B. Tyrosine hydroxylase

    Tyrosine hydroxylase is the rate-limiting step in catecholamine synthesis.

  • C. Homogentisate 1,2-dioxygenase (correct)

    Correct. Homogentisic acid accumulates, darkens the urine and stains cartilage.

  • D. Fumarylacetoacetate hydrolase

    Fumarylacetoacetate hydrolase deficiency causes hereditary tyrosinaemia type I with liver failure.

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, Homogentisate 1,2-dioxygenase. Alkaptonuria is an autosomal recessive deficiency of homogentisate oxidase (dioxygenase).

Why is "Tyrosinase" not the answer?

Tyrosinase deficiency causes oculocutaneous albinism.

Why is "Tyrosine hydroxylase" not the answer?

Tyrosine hydroxylase is the rate-limiting step in catecholamine synthesis.

Why is "Fumarylacetoacetate hydrolase" not the answer?

Fumarylacetoacetate hydrolase deficiency causes hereditary tyrosinaemia type I with liver failure.

What concept does this question test?

It tests Amino acid metabolism: tyrosine in Biochemistry. Homogentisic acid accumulates, oxidises and polymerises to a pigment that deposits in cartilage (ochronosis) and causes early degenerative arthritis. Urine darkens on standing or with alkali.

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: tyrosine?

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.