Nerve and muscle: Guillain-Barre conduction: 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: Guillain-Barre conduction

Sample item

A patient with ascending weakness after a diarrhoeal illness has absent reflexes and slowed conduction velocities with conduction block on nerve testing. Why does segmental demyelination slow and block conduction?

ASodium channels are lost from the axon hillockBThe axon loses its resting potential permanentlyCAxoplasmic resistance falls and the length constant risesDCurrent leaks across newly exposed internodal membrane, so the next node may not reach threshold

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

The answer and the reasoning

D. Current leaks across newly exposed internodal membrane, so the next node may not reach threshold

Myelin raises membrane resistance and lowers capacitance, letting current flow to the next node. When it is lost, current leaks across the exposed membrane and charging the membrane is slower, so the next node reaches threshold late or not at all. This causes slowed conduction and block.

Option by option

A. Sodium channels are lost from the axon hillock
Sodium channel loss at the hillock does not explain demyelination.
B. The axon loses its resting potential permanently
The resting potential is preserved in the early demyelinated axon.
C. Axoplasmic resistance falls and the length constant rises
Demyelination reduces the length constant by increasing membrane leak.
D. Current leaks across newly exposed internodal membrane, so the next node may not reach threshold (correct)
Correct. Leaky exposed internodes dissipate the current needed to reach the next node.

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 D, Current leaks across newly exposed internodal membrane, so the next node may not reach threshold. Myelin raises membrane resistance and lowers capacitance, letting current flow to the next node.

Why is "Sodium channels are lost from the axon hillock" not the answer?

Sodium channel loss at the hillock does not explain demyelination.

Why is "The axon loses its resting potential permanently" not the answer?

The resting potential is preserved in the early demyelinated axon.

Why is "Axoplasmic resistance falls and the length constant rises" not the answer?

Demyelination reduces the length constant by increasing membrane leak.

What concept does this question test?

It tests Nerve and muscle: Guillain-Barre conduction in Physiology. When it is lost, current leaks across the exposed membrane and charging the membrane is slower, so the next node reaches threshold late or not at all. This causes slowed conduction and block.

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: Guillain-Barre conduction?

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.