Lipid metabolism: primary carnitine deficiency: 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 · Lipid metabolism: primary carnitine deficiency

Sample item

A 2-year-old has dilated cardiomyopathy, muscle weakness and fasting hypoketotic hypoglycaemia. Plasma free carnitine is very low with excessive urinary carnitine loss, and he improves with oral carnitine. Which defect is most likely?

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

The answer and the reasoning

A. Plasma membrane carnitine transporter (OCTN2)

Primary carnitine deficiency results from a defective plasma membrane carnitine transporter (OCTN2), so carnitine is lost in urine and cannot enter muscle and heart. Long-chain fatty acids cannot be transported into mitochondria, causing hypoketotic hypoglycaemia and cardiomyopathy that respond to supplementation.

Option by option

  • A. Plasma membrane carnitine transporter (OCTN2) (correct)

    Correct. A defective OCTN2 transporter causes urinary carnitine wasting and low tissue carnitine.

  • B. Carnitine palmitoyltransferase I

    CPT-I deficiency gives hepatic hypoketotic hypoglycaemia with normal or high plasma carnitine.

  • C. Carnitine palmitoyltransferase II

    CPT-II deficiency causes exertional myoglobinuria with raised acylcarnitines and normal free carnitine.

  • D. Carnitine-acylcarnitine translocase

    Translocase deficiency presents with severe neonatal arrhythmia and raised long-chain acylcarnitines, not low free carnitine.

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, Plasma membrane carnitine transporter (OCTN2). Primary carnitine deficiency results from a defective plasma membrane carnitine transporter (OCTN2), so carnitine is lost in urine and cannot enter muscle and heart.

Why is "Carnitine palmitoyltransferase I" not the answer?

CPT-I deficiency gives hepatic hypoketotic hypoglycaemia with normal or high plasma carnitine.

Why is "Carnitine palmitoyltransferase II" not the answer?

CPT-II deficiency causes exertional myoglobinuria with raised acylcarnitines and normal free carnitine.

Why is "Carnitine-acylcarnitine translocase" not the answer?

Translocase deficiency presents with severe neonatal arrhythmia and raised long-chain acylcarnitines, not low free carnitine.

What concept does this question test?

It tests Lipid metabolism: primary carnitine deficiency in Biochemistry. Long-chain fatty acids cannot be transported into mitochondria, causing hypoketotic hypoglycaemia and cardiomyopathy that respond to supplementation.

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 Lipid metabolism: primary carnitine deficiency?

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.