Nerve and muscle: carnitine shuttle in muscle: practice question

A Physiology 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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Physiology · Nerve and muscle: carnitine shuttle in muscle

Sample item

A 22-year-old has muscle pain and dark urine after prolonged exercise during a fast. Venous lactate rises normally during ischaemic forearm exercise, and glycogen stores are normal. Which process is deficient?

ABreakdown of glycogen by muscle phosphorylaseBCarnitine-mediated entry of long-chain fatty acids into mitochondriaCPyruvate conversion to lactate in the cytosolDOxidation of ketone bodies in cardiac muscle

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

The answer and the reasoning

B. Carnitine-mediated entry of long-chain fatty acids into mitochondria

Long-chain fatty acids need carnitine palmitoyltransferase to enter mitochondria, and this is the main fuel in prolonged exercise and fasting. Deficiency of CPT II causes exertional rhabdomyolysis with normal glycolysis, so lactate rises normally. In glycogen phosphorylase deficiency, lactate fails to rise.

Option by option

A. Breakdown of glycogen by muscle phosphorylase
A phosphorylase defect would prevent the lactate rise in the forearm test.
B. Carnitine-mediated entry of long-chain fatty acids into mitochondria (correct)
Correct. Impaired fatty acid entry causes problems in prolonged exercise or fasting with intact glycolysis.
C. Pyruvate conversion to lactate in the cytosol
Lactate production was normal.
D. Oxidation of ketone bodies in cardiac muscle
Ketone oxidation in the heart is not the cause of this muscle presentation.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, Carnitine-mediated entry of long-chain fatty acids into mitochondria. Long-chain fatty acids need carnitine palmitoyltransferase to enter mitochondria, and this is the main fuel in prolonged exercise and fasting.

Why is "Breakdown of glycogen by muscle phosphorylase" not the answer?

A phosphorylase defect would prevent the lactate rise in the forearm test.

Why is "Pyruvate conversion to lactate in the cytosol" not the answer?

Lactate production was normal.

Why is "Oxidation of ketone bodies in cardiac muscle" not the answer?

Ketone oxidation in the heart is not the cause of this muscle presentation.

What concept does this question test?

It tests Nerve and muscle: carnitine shuttle in muscle in Physiology. Deficiency of CPT II causes exertional rhabdomyolysis with normal glycolysis, so lactate rises normally. In glycogen phosphorylase deficiency, lactate fails to rise.

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 Nerve and muscle: carnitine shuttle in muscle?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.