Cell injury and adaptation: ochronosis: practice question

A Pathology 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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Pathology · Cell injury and adaptation: ochronosis

Sample item

A 45-year-old man has urine that turns dark on standing, bluish-black pigmentation of the ear cartilage and sclerae, and degenerative arthritis of the spine. Which enzyme is deficient?

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

The answer and the reasoning

D. Homogentisate oxidase

In alkaptonuria, homogentisate oxidase deficiency lets homogentisic acid accumulate. It oxidises and polymerises to a dark pigment that binds collagen in cartilage, producing ochronosis and early arthropathy. Urine darkens on standing or alkalinisation.

Option by option

  • A. Fumarylacetoacetate hydrolase

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

  • B. Tyrosinase

    Tyrosinase deficiency causes oculocutaneous albinism, which lacks pigment.

  • C. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency causes phenylketonuria, with a mousy odour and fair skin.

  • D. Homogentisate oxidase (correct)

    Correct. Homogentisic acid accumulates and deposits in connective tissue as dark pigment.

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 Pathology question

What is the answer to this Pathology question?

The best answer is D, Homogentisate oxidase. In alkaptonuria, homogentisate oxidase deficiency lets homogentisic acid accumulate.

Why is "Fumarylacetoacetate hydrolase" not the answer?

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

Why is "Tyrosinase" not the answer?

Tyrosinase deficiency causes oculocutaneous albinism, which lacks pigment.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria, with a mousy odour and fair skin.

What concept does this question test?

It tests Cell injury and adaptation: ochronosis in Pathology. It oxidises and polymerises to a dark pigment that binds collagen in cartilage, producing ochronosis and early arthropathy. Urine darkens on standing or alkalinisation.

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 Cell injury and adaptation: ochronosis?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.