Pharmacology: alfentanil 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 · Pharmacology: alfentanil onset

Sample item

Alfentanil has a faster onset than fentanyl, although it is less lipid soluble. What is the main explanation?

AA higher protein bindingBA longer elimination half-lifeCA pKa of about 6.5, so most of the drug is unionised at physiological pHDA larger volume of distribution

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

The answer and the reasoning

C. A pKa of about 6.5, so most of the drug is unionised at physiological pH

Alfentanil has a pKa of about 6.5, so around 90% is unionised at pH 7.4 and available to cross the blood-brain barrier quickly, with onset in about a minute. Its small volume of distribution also shortens its duration. Fentanyl has a pKa of about 8.4 and is largely ionised at physiological pH.

Option by option

A. A higher protein binding
Protein binding does not explain faster onset.
B. A longer elimination half-life
A shorter, not longer, half-life contributes to its profile.
C. A pKa of about 6.5, so most of the drug is unionised at physiological pH (correct)
Correct. A low pKa gives a high unionised fraction at body pH.
D. A larger volume of distribution
Alfentanil has a smaller, not larger, volume of distribution.

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 C, A pKa of about 6.5, so most of the drug is unionised at physiological pH. Alfentanil has a pKa of about 6.

Why is "A higher protein binding" not the answer?

Protein binding does not explain faster onset.

Why is "A longer elimination half-life" not the answer?

A shorter, not longer, half-life contributes to its profile.

Why is "A larger volume of distribution" not the answer?

Alfentanil has a smaller, not larger, volume of distribution.

What concept does this question test?

It tests Pharmacology: alfentanil onset in Anaesthesia. 5, so around 90% is unionised at pH 7.4 and available to cross the blood-brain barrier quickly, with onset in about a minute. Its small volume of distribution also shortens its duration. Fentanyl has a pKa of about 8.4 and is largely ionised at physiological pH.

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 Pharmacology: alfentanil 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.