Nerve physiology: fibre susceptibility: 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 physiology: fibre susceptibility

Sample item

After a long period of pressure on the upper arm during deep sleep, a man wakes with wrist drop and loss of position sense in the hand, but pain sensation is relatively preserved. Which property of nerve fibres explains this pattern?

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

The answer and the reasoning

A. Large myelinated A fibres are more susceptible to pressure than small C fibres

Compression affects fibres in a size-dependent manner: large-diameter, fast A fibres carrying motor and proprioceptive impulses are blocked first, while small C fibres carrying slow pain are blocked last. With local anaesthetics the order is reversed, because small fibres are blocked first. This is why neuropraxia from pressure affects motor power and joint position before pain.

Option by option

  • A. Large myelinated A fibres are more susceptible to pressure than small C fibres (correct)

    Large A fibres are the most pressure-sensitive, so motor and proprioceptive loss appear first.

  • B. C fibres are more susceptible to pressure and so are blocked first

    C fibres are the least sensitive to pressure; they are the most sensitive to local anaesthetics.

  • C. Motor fibres are unmyelinated and so are damaged by mechanical stress

    Motor fibres to skeletal muscle are large and myelinated, not unmyelinated.

  • D. Pain fibres conduct fastest and recover first after compression

    Pain fibres conduct slowly, being thin; the fastest are large A-alpha fibres.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, Large myelinated A fibres are more susceptible to pressure than small C fibres. Compression affects fibres in a size-dependent manner: large-diameter, fast A fibres carrying motor and proprioceptive impulses are blocked first, while small C fibres carrying slow pain are blocked last.

Why is "C fibres are more susceptible to pressure and so are blocked first" not the answer?

C fibres are the least sensitive to pressure; they are the most sensitive to local anaesthetics.

Why is "Motor fibres are unmyelinated and so are damaged by mechanical stress" not the answer?

Motor fibres to skeletal muscle are large and myelinated, not unmyelinated.

Why is "Pain fibres conduct fastest and recover first after compression" not the answer?

Pain fibres conduct slowly, being thin; the fastest are large A-alpha fibres.

What concept does this question test?

It tests Nerve physiology: fibre susceptibility in Physiology. With local anaesthetics the order is reversed, because small fibres are blocked first. This is why neuropraxia from pressure affects motor power and joint position before pain.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 physiology: fibre susceptibility?

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.