Muscle relaxants: potency and speed of onset: practice question

A Anaesthesia 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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Anaesthesia · Muscle relaxants: potency and speed of onset

Sample item

Rocuronium has a faster onset than vecuronium, although both are aminosteroid relaxants. What is the main reason?

AFaster metabolism in plasma before reaching muscleBLower potency, so a larger number of molecules is givenCGreater lipid solubility enabling quicker entry into muscleDHigher potency, so a smaller dose reaches the junction

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

The answer and the reasoning

B. Lower potency, so a larger number of molecules is given

Among non-depolarising relaxants, onset is inversely related to potency. A less potent drug is given in a larger molar dose, so the concentration gradient driving drug into the biophase is steeper and more receptors are occupied quickly. That is why rocuronium acts faster than vecuronium.

Option by option

A. Faster metabolism in plasma before reaching muscle
Rocuronium is not broken down in plasma; it is cleared by liver and kidney.
B. Lower potency, so a larger number of molecules is given (correct)
Correct. Lower potency needs a bigger dose, and the higher concentration speeds onset.
C. Greater lipid solubility enabling quicker entry into muscle
Relaxants are charged quaternary molecules and enter the biophase from plasma, not by lipid solubility.
D. Higher potency, so a smaller dose reaches the junction
A higher potency drug is given in smaller doses and tends to have a slower onset.

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 Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is B, Lower potency, so a larger number of molecules is given. Among non-depolarising relaxants, onset is inversely related to potency.

Why is "Faster metabolism in plasma before reaching muscle" not the answer?

Rocuronium is not broken down in plasma; it is cleared by liver and kidney.

Why is "Greater lipid solubility enabling quicker entry into muscle" not the answer?

Relaxants are charged quaternary molecules and enter the biophase from plasma, not by lipid solubility.

Why is "Higher potency, so a smaller dose reaches the junction" not the answer?

A higher potency drug is given in smaller doses and tends to have a slower onset.

What concept does this question test?

It tests Muscle relaxants: potency and speed of onset in Anaesthesia. A less potent drug is given in a larger molar dose, so the concentration gradient driving drug into the biophase is steeper and more receptors are occupied quickly. That is why rocuronium acts faster than vecuronium.

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 relaxants: potency and speed of onset?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia 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.