Rheumatology: alkaptonuria: practice question

A General Medicine 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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General Medicine · Rheumatology: alkaptonuria

Sample item

A 45-year-old man has low back stiffness, bluish-black discolouration of the ear cartilage and sclerae, and urine that darkens on 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 1,2-dioxygenase. Accumulated homogentisic acid oxidises to a dark pigment deposited in cartilage (ochronosis), causing early spondyloarthropathy and degenerative joint disease, and urine darkens on standing or alkalinisation.

Option by option

  • A. Phenylalanine hydroxylase

    Phenylalanine hydroxylase deficiency causes phenylketonuria.

  • B. Fumarylacetoacetate hydrolase

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

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

    Correct. Absent homogentisate oxidase leads to homogentisic aciduria and ochronosis.

  • D. Tyrosinase

    Tyrosinase deficiency causes oculocutaneous albinism.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is C, Homogentisate 1,2-dioxygenase. Alkaptonuria is an autosomal recessive deficiency of homogentisate 1,2-dioxygenase.

Why is "Phenylalanine hydroxylase" not the answer?

Phenylalanine hydroxylase deficiency causes phenylketonuria.

Why is "Fumarylacetoacetate hydrolase" not the answer?

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

Why is "Tyrosinase" not the answer?

Tyrosinase deficiency causes oculocutaneous albinism.

What concept does this question test?

It tests Rheumatology: alkaptonuria in General Medicine. Accumulated homogentisic acid oxidises to a dark pigment deposited in cartilage (ochronosis), causing early spondyloarthropathy and degenerative joint disease, and 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 Rheumatology: alkaptonuria?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.