Nerve and muscle: myotonia congenita: 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: myotonia congenita

Sample item

A 15-year-old has difficulty releasing his grip after a handshake, which improves with repeated movement. EMG shows repetitive discharges that wax and wane. Which channel abnormality is likely?

AReduced nicotinic receptor number at the end plateBReduced skeletal muscle chloride conductanceCFailed sodium channel inactivation, worse with exercise and coldDRyanodine receptor leak, causing heat-triggered rigidity

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

The answer and the reasoning

B. Reduced skeletal muscle chloride conductance

Mutations in the skeletal muscle chloride channel CLCN1 lower resting chloride conductance. The membrane then becomes hyperexcitable, because potassium accumulating in the T tubules depolarises it and triggers repetitive firing, delaying relaxation. The stiffness is eased by warm-up. Paramyotonia, due to sodium channel mutations, instead worsens with repetition and cold.

Option by option

A. Reduced nicotinic receptor number at the end plate
Fewer nicotinic receptors would cause fatigable weakness, not delayed relaxation.
B. Reduced skeletal muscle chloride conductance (correct)
Correct. Loss of chloride conductance leaves the membrane prone to repetitive firing that eases with exercise.
C. Failed sodium channel inactivation, worse with exercise and cold
Failed sodium channel inactivation describes paramyotonia, which worsens with repeated use and cold.
D. Ryanodine receptor leak, causing heat-triggered rigidity
A ryanodine receptor leak is the basis of malignant hyperthermia, not myotonia.

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, Reduced skeletal muscle chloride conductance. Mutations in the skeletal muscle chloride channel CLCN1 lower resting chloride conductance.

Why is "Reduced nicotinic receptor number at the end plate" not the answer?

Fewer nicotinic receptors would cause fatigable weakness, not delayed relaxation.

Why is "Failed sodium channel inactivation, worse with exercise and cold" not the answer?

Failed sodium channel inactivation describes paramyotonia, which worsens with repeated use and cold.

Why is "Ryanodine receptor leak, causing heat-triggered rigidity" not the answer?

A ryanodine receptor leak is the basis of malignant hyperthermia, not myotonia.

What concept does this question test?

It tests Nerve and muscle: myotonia congenita in Physiology. The membrane then becomes hyperexcitable, because potassium accumulating in the T tubules depolarises it and triggers repetitive firing, delaying relaxation. The stiffness is eased by warm-up. Paramyotonia, due to sodium channel mutations, instead worsens with repetition and cold.

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: myotonia congenita?

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.