Joint disorders: alkaptonuric ochronosis: practice question

A Orthopaedics 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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Orthopaedics · Joint disorders: alkaptonuric ochronosis

Sample item

A 45-year-old has low back stiffness, early large joint arthritis and dark pigmentation of the ear cartilage. His urine turns black on standing. Radiographs show calcified intervertebral discs. What is the enzyme deficiency?

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

The answer and the reasoning

C. Homogentisic acid oxidase

Alkaptonuria results from homogentisic acid oxidase deficiency, so homogentisic acid accumulates and polymerises into pigment in cartilage, which is called ochronosis. It leads to early spondylosis with calcified discs and arthritis of large joints. Urine darkens on standing or alkalinisation.

Option by option

  • A. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency causes phenylketonuria.

  • B. Iduronate sulphatase

    Iduronate sulphatase deficiency causes Hunter syndrome.

  • C. Homogentisic acid oxidase (correct)

    Correct. Homogentisic acid oxidase deficiency causes alkaptonuria.

  • D. Glucocerebrosidase

    Glucocerebrosidase deficiency causes Gaucher disease.

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 Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is C, Homogentisic acid oxidase. Alkaptonuria results from homogentisic acid oxidase deficiency, so homogentisic acid accumulates and polymerises into pigment in cartilage, which is called ochronosis.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria.

Why is "Iduronate sulphatase" not the answer?

Iduronate sulphatase deficiency causes Hunter syndrome.

Why is "Glucocerebrosidase" not the answer?

Glucocerebrosidase deficiency causes Gaucher disease.

What concept does this question test?

It tests Joint disorders: alkaptonuric ochronosis in Orthopaedics. It leads to early spondylosis with calcified discs and arthritis of large joints. 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 Joint disorders: alkaptonuric ochronosis?

Pair each fracture with its classic complication and nerve injured. Then use the flashcards and the other Orthopaedics 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.