Nerve and muscle: local anaesthetic and tissue pH: 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: local anaesthetic and tissue pH

Sample item

A surgeon infiltrates lidocaine (pKa 7.9) into an acutely infected, acidic abscess and the block is poor. Which mechanism best explains this?

AThe sodium channels are absent in inflamed nervesBAcidic pH increases the unionised fraction, so the drug cannot bind the sodium channelCAcidic pH raises the ionised fraction, so less drug crosses the nerve membrane in the lipid-soluble formDAcidosis raises the pKa of lidocaine to 9

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

The answer and the reasoning

C. Acidic pH raises the ionised fraction, so less drug crosses the nerve membrane in the lipid-soluble form

Local anaesthetics are weak bases that cross the nerve membrane in the unionised, lipid-soluble form and then act from inside the cell in the ionised form. In acidic tissue a larger proportion is ionised, so less penetrates the axon and the block is weak or slow. Alkalinising the solution with bicarbonate speeds onset.

Option by option

A. The sodium channels are absent in inflamed nerves
Sodium channels are present in inflamed nerves, and the problem is access of the drug.
B. Acidic pH increases the unionised fraction, so the drug cannot bind the sodium channel
An acid environment increases the ionised fraction, not the unionised one.
C. Acidic pH raises the ionised fraction, so less drug crosses the nerve membrane in the lipid-soluble form (correct)
Correct. More ionised drug in acid tissue means less membrane penetration.
D. Acidosis raises the pKa of lidocaine to 9
The pKa is a property of the molecule and does not change with tissue pH.

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 C, Acidic pH raises the ionised fraction, so less drug crosses the nerve membrane in the lipid-soluble form. Local anaesthetics are weak bases that cross the nerve membrane in the unionised, lipid-soluble form and then act from inside the cell in the ionised form.

Why is "The sodium channels are absent in inflamed nerves" not the answer?

Sodium channels are present in inflamed nerves, and the problem is access of the drug.

Why is "Acidic pH increases the unionised fraction, so the drug cannot bind t…" not the answer?

An acid environment increases the ionised fraction, not the unionised one.

Why is "Acidosis raises the pKa of lidocaine to 9" not the answer?

The pKa is a property of the molecule and does not change with tissue pH.

What concept does this question test?

It tests Nerve and muscle: local anaesthetic and tissue pH in Physiology. In acidic tissue a larger proportion is ionised, so less penetrates the axon and the block is weak or slow. Alkalinising the solution with bicarbonate speeds onset.

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: local anaesthetic and tissue pH?

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.