Fatty acid oxidation: carnitine palmitoyltransferase II 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 · Fatty acid oxidation: carnitine palmitoyltransferase II deficiency

Sample item

A 19-year-old man has severe muscle pain and dark urine after a long run following an overnight fast and a viral illness. CK is 25,000 U/L. In the ischaemic forearm test, lactate rises normally, and he has no cramps with brief intense exercise. Which diagnosis is most likely?

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

The answer and the reasoning

B. Carnitine palmitoyltransferase II deficiency

In CPT II deficiency, long-chain fatty acids cannot be oxidised in muscle mitochondria, so prolonged exercise, fasting or fever provokes rhabdomyolysis with myoglobinuria, while glycogenolysis is intact. The normal lactate rise in the ischaemic forearm test excludes the glycogen storage myopathies. Management is a high-carbohydrate diet and avoiding prolonged fasting and strenuous exercise.

Option by option

  • A. McArdle disease

    McArdle disease blocks glycogenolysis, so lactate fails to rise and cramps start within minutes of intense exercise.

  • B. Carnitine palmitoyltransferase II deficiency (correct)

    A normal lactate response with exertional rhabdomyolysis after fasting indicates a fat oxidation defect, CPT II.

  • C. Tarui disease

    Tarui disease, muscle phosphofructokinase deficiency, also blocks the lactate rise and gives early exercise intolerance.

  • D. Pompe disease

    Pompe disease causes lysosomal glycogen storage with progressive weakness and a normal lactate rise but no exertional myoglobinuria.

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 B, Carnitine palmitoyltransferase II deficiency. In CPT II deficiency, long-chain fatty acids cannot be oxidised in muscle mitochondria, so prolonged exercise, fasting or fever provokes rhabdomyolysis with myoglobinuria, while glycogenolysis is intact.

Why is "McArdle disease" not the answer?

McArdle disease blocks glycogenolysis, so lactate fails to rise and cramps start within minutes of intense exercise.

Why is "Tarui disease" not the answer?

Tarui disease, muscle phosphofructokinase deficiency, also blocks the lactate rise and gives early exercise intolerance.

Why is "Pompe disease" not the answer?

Pompe disease causes lysosomal glycogen storage with progressive weakness and a normal lactate rise but no exertional myoglobinuria.

What concept does this question test?

It tests Fatty acid oxidation: carnitine palmitoyltransferase II deficiency in Biochemistry. The normal lactate rise in the ischaemic forearm test excludes the glycogen storage myopathies. Management is a high-carbohydrate diet and avoiding prolonged fasting and strenuous exercise.

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 Fatty acid oxidation: carnitine palmitoyltransferase II 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.