Nerve and muscle: use-dependent sodium channel block: 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: use-dependent sodium channel block

Sample item

Lidocaine given intravenously suppresses ventricular tachycardia in ischaemic myocardium, but has little effect on the normal atrial rhythm at a rate of 70 per minute. Which property of its action explains this selectivity?

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

The answer and the reasoning

B. It binds preferentially to open and inactivated sodium channels, so block accumulates in rapidly firing or depolarised tissue

Local anaesthetics and class I antiarrhythmics bind sodium channels mainly in the open and inactivated states and dissociate slowly in the resting state. Fast firing and depolarised, ischaemic tissue spend more time in these states, so use-dependent block builds up there while normal slow rhythms recover between beats.

Option by option

  • A. It blocks voltage-gated potassium channels and prolongs repolarisation in all cells equally

    Potassium channel block is the mechanism of class III drugs such as amiodarone.

  • B. It binds preferentially to open and inactivated sodium channels, so block accumulates in rapidly firing or depolarised tissue (correct)

    Correct. State-dependent binding gives use-dependent block.

  • C. It blocks L-type calcium channels in the atrioventricular node

    Calcium channel block with slowed AV nodal conduction is the action of verapamil, not lidocaine.

  • D. It blocks only resting-state sodium channels, so it works best at slow rates

    Preference for resting channels would make block strongest at slow rates, the opposite of what is seen.

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 B, It binds preferentially to open and inactivated sodium channels, so block accumulates in rapidly firing or depolarised tissue. Local anaesthetics and class I antiarrhythmics bind sodium channels mainly in the open and inactivated states and dissociate slowly in the resting state.

Why is "It blocks voltage-gated potassium channels and prolongs repolarisatio…" not the answer?

Potassium channel block is the mechanism of class III drugs such as amiodarone.

Why is "It blocks L-type calcium channels in the atrioventricular node" not the answer?

Calcium channel block with slowed AV nodal conduction is the action of verapamil, not lidocaine.

Why is "It blocks only resting-state sodium channels, so it works best at slo…" not the answer?

Preference for resting channels would make block strongest at slow rates, the opposite of what is seen.

What concept does this question test?

It tests Nerve and muscle: use-dependent sodium channel block in Physiology. Fast firing and depolarised, ischaemic tissue spend more time in these states, so use-dependent block builds up there while normal slow rhythms recover between beats.

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 and muscle: use-dependent sodium channel block?

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.