Nerve and muscle: potassium channels in demyelinated axons: 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: potassium channels in demyelinated axons

Sample item

In multiple sclerosis, fampridine (4-aminopyridine) can improve walking speed. Which action explains the benefit?

AIt blocks potassium channels exposed in demyelinated axons, restoring action potential conductionBIt stimulates remyelination by oligodendrocytesCIt blocks NMDA receptors on motor neuronesDIt opens sodium channels at the nodes of Ranvier

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

The answer and the reasoning

A. It blocks potassium channels exposed in demyelinated axons, restoring action potential conduction

When myelin is lost, voltage-gated potassium channels normally hidden under the sheath become exposed and leak current, which can block conduction. Blocking them prolongs the action potential and helps it reach the next node, improving conduction in surviving demyelinated axons.

Option by option

A. It blocks potassium channels exposed in demyelinated axons, restoring action potential conduction (correct)
Correct. Blocking exposed potassium channels restores conduction.
B. It stimulates remyelination by oligodendrocytes
It does not remyelinate; it improves conduction in axons that remain demyelinated.
C. It blocks NMDA receptors on motor neurones
NMDA receptor blockade is not its mechanism.
D. It opens sodium channels at the nodes of Ranvier
Fampridine does not open sodium channels; it blocks potassium channels.

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 A, It blocks potassium channels exposed in demyelinated axons, restoring action potential conduction. When myelin is lost, voltage-gated potassium channels normally hidden under the sheath become exposed and leak current, which can block conduction.

Why is "It stimulates remyelination by oligodendrocytes" not the answer?

It does not remyelinate; it improves conduction in axons that remain demyelinated.

Why is "It blocks NMDA receptors on motor neurones" not the answer?

NMDA receptor blockade is not its mechanism.

Why is "It opens sodium channels at the nodes of Ranvier" not the answer?

Fampridine does not open sodium channels; it blocks potassium channels.

What concept does this question test?

It tests Nerve and muscle: potassium channels in demyelinated axons in Physiology. Blocking them prolongs the action potential and helps it reach the next node, improving conduction in surviving demyelinated axons.

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: potassium channels in demyelinated axons?

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.