Fatty acid oxidation: Refsum disease: 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.

Try it

Biochemistry · Fatty acid oxidation: Refsum disease

Sample item

A 30-year-old man has night blindness from retinitis pigmentosa, peripheral neuropathy, cerebellar ataxia, sensorineural deafness and dry scaly skin. Serum phytanic acid is markedly raised. Which metabolic pathway is defective?

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

The answer and the reasoning

A. Peroxisomal alpha-oxidation of branched-chain fatty acids

Phytanic acid, derived from chlorophyll in the diet and found in dairy fat and ruminant meat, has a methyl group at the beta carbon that blocks beta-oxidation. It is first shortened by alpha-oxidation using phytanoyl-CoA hydroxylase in peroxisomes. Deficiency of this enzyme causes Refsum disease, treated by restricting dietary phytanic acid.

Option by option

  • A. Peroxisomal alpha-oxidation of branched-chain fatty acids (correct)

    A block in alpha-oxidation allows phytanic acid to accumulate in nerve, retina and skin.

  • B. Microsomal omega-oxidation of fatty acids

    Omega-oxidation is a minor pathway that produces dicarboxylic acids and is not the cause of phytanate storage.

  • C. Peroxisomal beta-oxidation of very long chain fatty acids

    Defective peroxisomal beta-oxidation of very long chain fatty acids causes X-linked adrenoleukodystrophy, with raised VLCFA rather than phytanate.

  • D. Mitochondrial beta-oxidation of medium-chain fatty acids

    Defective medium-chain beta-oxidation presents with hypoketotic hypoglycaemia and not retinitis pigmentosa with raised phytanate.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Peroxisomal alpha-oxidation of branched-chain fatty acids. Phytanic acid, derived from chlorophyll in the diet and found in dairy fat and ruminant meat, has a methyl group at the beta carbon that blocks beta-oxidation.

Why is "Microsomal omega-oxidation of fatty acids" not the answer?

Omega-oxidation is a minor pathway that produces dicarboxylic acids and is not the cause of phytanate storage.

Why is "Peroxisomal beta-oxidation of very long chain fatty acids" not the answer?

Defective peroxisomal beta-oxidation of very long chain fatty acids causes X-linked adrenoleukodystrophy, with raised VLCFA rather than phytanate.

Why is "Mitochondrial beta-oxidation of medium-chain fatty acids" not the answer?

Defective medium-chain beta-oxidation presents with hypoketotic hypoglycaemia and not retinitis pigmentosa with raised phytanate.

What concept does this question test?

It tests Fatty acid oxidation: Refsum disease in Biochemistry. It is first shortened by alpha-oxidation using phytanoyl-CoA hydroxylase in peroxisomes. Deficiency of this enzyme causes Refsum disease, treated by restricting dietary phytanic acid.

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 Fatty acid oxidation: Refsum disease?

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.