Peroxisomal disorders: X-linked adrenoleukodystrophy: 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 · Peroxisomal disorders: X-linked adrenoleukodystrophy

Sample item

An 8-year-old boy has declining school performance, visual difficulty and skin hyperpigmentation. His morning cortisol is low with a very high ACTH, and MRI shows posterior white matter demyelination. Which finding would confirm the diagnosis?

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

The answer and the reasoning

A. Raised plasma very long chain fatty acids (C26:0) from an ABCD1 transporter defect

X-linked adrenoleukodystrophy results from loss of ABCD1, the peroxisomal membrane transporter that brings very long chain fatty acids in for beta-oxidation. They accumulate in the adrenal cortex (primary adrenal insufficiency, with high ACTH and pigmentation) and in the cerebral white matter, producing demyelination. Plasma C26:0 and the C26:C22 ratio are raised and ABCD1 sequencing confirms it.

Option by option

  • A. Raised plasma very long chain fatty acids (C26:0) from an ABCD1 transporter defect (correct)

    Raised C26:0 with the ABCD1 defect explains both adrenal failure and demyelination.

  • B. Raised urinary sulfatide from an arylsulfatase A defect

    Sulfatiduria marks metachromatic leukodystrophy, which does not cause adrenal insufficiency.

  • C. Raised plasma C8 acylcarnitine from medium-chain acyl-CoA dehydrogenase deficiency

    Raised C8 acylcarnitine indicates MCAD deficiency, which causes hypoketotic hypoglycaemia and no demyelination.

  • D. Raised plasma phytanic acid from a phytanoyl-CoA hydroxylase defect

    Phytanic acid accumulates in Refsum disease, which gives retinitis pigmentosa, ataxia and neuropathy without adrenal 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 A, Raised plasma very long chain fatty acids (C26:0) from an ABCD1 transporter defect. X-linked adrenoleukodystrophy results from loss of ABCD1, the peroxisomal membrane transporter that brings very long chain fatty acids in for beta-oxidation.

Why is "Raised urinary sulfatide from an arylsulfatase A defect" not the answer?

Sulfatiduria marks metachromatic leukodystrophy, which does not cause adrenal insufficiency.

Why is "Raised plasma C8 acylcarnitine from medium-chain acyl-CoA dehydrogena…" not the answer?

Raised C8 acylcarnitine indicates MCAD deficiency, which causes hypoketotic hypoglycaemia and no demyelination.

Why is "Raised plasma phytanic acid from a phytanoyl-CoA hydroxylase defect" not the answer?

Phytanic acid accumulates in Refsum disease, which gives retinitis pigmentosa, ataxia and neuropathy without adrenal failure.

What concept does this question test?

It tests Peroxisomal disorders: X-linked adrenoleukodystrophy in Biochemistry. They accumulate in the adrenal cortex (primary adrenal insufficiency, with high ACTH and pigmentation) and in the cerebral white matter, producing demyelination. Plasma C26:0 and the C26:C22 ratio are raised and ABCD1 sequencing confirms it.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Peroxisomal disorders: X-linked adrenoleukodystrophy?

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.