Muscle physiology: electromyography in myopathy: 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 · Muscle physiology: electromyography in myopathy

Sample item

A 10-year-old boy with proximal muscle weakness and a very high creatine kinase has needle electromyography showing small, short, polyphasic motor unit potentials with early recruitment. Why are the potentials small?

AMotor axons are blocked and fail to conductBEach motor unit contains fewer functioning muscle fibres, so it generates a smaller summed potentialCAcetylcholine release is reduced at the neuromuscular junctionDSurviving axons have sprouted and enlarged each motor unit

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

The answer and the reasoning

B. Each motor unit contains fewer functioning muscle fibres, so it generates a smaller summed potential

The motor unit potential is the sum of potentials of the muscle fibres in one unit. In primary muscle disease fibres are lost or dysfunctional, so each unit generates a small, brief, polyphasic potential, and more units are recruited early to produce a given force. Neurogenic disease gives large, long potentials with reduced recruitment.

Option by option

A. Motor axons are blocked and fail to conduct
A conduction block would reduce the number of units firing, not shrink individual units.
B. Each motor unit contains fewer functioning muscle fibres, so it generates a smaller summed potential (correct)
Correct. Fewer fibres per unit reduce the summed amplitude.
C. Acetylcholine release is reduced at the neuromuscular junction
Junctional failure causes variable amplitude and decrement, not uniformly small polyphasic units.
D. Surviving axons have sprouted and enlarged each motor unit
Reinnervation enlarging motor units produces large, long potentials, as in chronic neuropathy.

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, Each motor unit contains fewer functioning muscle fibres, so it generates a smaller summed potential. The motor unit potential is the sum of potentials of the muscle fibres in one unit.

Why is "Motor axons are blocked and fail to conduct" not the answer?

A conduction block would reduce the number of units firing, not shrink individual units.

Why is "Acetylcholine release is reduced at the neuromuscular junction" not the answer?

Junctional failure causes variable amplitude and decrement, not uniformly small polyphasic units.

Why is "Surviving axons have sprouted and enlarged each motor unit" not the answer?

Reinnervation enlarging motor units produces large, long potentials, as in chronic neuropathy.

What concept does this question test?

It tests Muscle physiology: electromyography in myopathy in Physiology. In primary muscle disease fibres are lost or dysfunctional, so each unit generates a small, brief, polyphasic potential, and more units are recruited early to produce a given force. Neurogenic disease gives large, long potentials with reduced recruitment.

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 Muscle physiology: electromyography in myopathy?

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.