General pharmacology: ion trapping of weak bases: practice question

A Pharmacology 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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Pharmacology · General pharmacology: ion trapping of weak bases

Sample item

A student wants to speed up the renal excretion of amphetamine, a weak base, after an overdose. Which change in urine is appropriate in principle?

AAcidify the urine so the drug is ionised and trapped in the tubuleBAlkalinise the urine to increase ionisation of the baseCAlkalinise the urine to reduce ionisation and promote excretionDIncrease urine volume only, since pH has no effect on a base

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

The answer and the reasoning

A. Acidify the urine so the drug is ionised and trapped in the tubule

A weak base is more ionised in acidic urine, and ionised molecules are not reabsorbed across the tubular epithelium, so excretion rises. Weak acids such as salicylate show the opposite pattern and are cleared faster in alkaline urine.

Option by option

A. Acidify the urine so the drug is ionised and trapped in the tubule (correct)
Correct. Acidification ionises a weak base and traps it in the lumen.
B. Alkalinise the urine to increase ionisation of the base
Alkalinisation would reduce ionisation of a base and increase its reabsorption.
C. Alkalinise the urine to reduce ionisation and promote excretion
Reducing ionisation favours reabsorption, which slows excretion of the base.
D. Increase urine volume only, since pH has no effect on a base
Urinary pH strongly alters the excretion of weak electrolytes.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, Acidify the urine so the drug is ionised and trapped in the tubule. A weak base is more ionised in acidic urine, and ionised molecules are not reabsorbed across the tubular epithelium, so excretion rises.

Why is "Alkalinise the urine to increase ionisation of the base" not the answer?

Alkalinisation would reduce ionisation of a base and increase its reabsorption.

Why is "Alkalinise the urine to reduce ionisation and promote excretion" not the answer?

Reducing ionisation favours reabsorption, which slows excretion of the base.

Why is "Increase urine volume only, since pH has no effect on a base" not the answer?

Urinary pH strongly alters the excretion of weak electrolytes.

What concept does this question test?

It tests General pharmacology: ion trapping of weak bases in Pharmacology. Weak acids such as salicylate show the opposite pattern and are cleared faster in alkaline urine.

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 General pharmacology: ion trapping of weak bases?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.