Pharmacokinetics: ion trapping: 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 · Pharmacokinetics: ion trapping

Sample item

A 20-year-old woman is brought in after ingesting a large number of aspirin tablets. She has tinnitus, rapid breathing and a mixed acid-base disturbance. Aspirin is a weak acid with a pKa of about 3.5. Why does alkalinising the urine with intravenous sodium bicarbonate hasten salicylate elimination?

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

The answer and the reasoning

C. Salicylate becomes more ionised in tubular fluid, so it cannot be passively reabsorbed

Only the non-ionised fraction of a weak electrolyte crosses the tubular epithelium. For a weak acid, raising the pH of tubular fluid above its pKa shifts it to the ionised form, which is trapped in the lumen and excreted. This ion trapping is the basis of urinary alkalinisation in salicylate and phenobarbitone poisoning.

Option by option

  • A. Salicylate becomes more lipid soluble, so it is secreted more actively into the tubule

    Ionisation reduces, not increases, lipid solubility, and passive back-diffusion is what is being prevented.

  • B. Glomerular filtration of salicylate rises because alkalinisation dilates the afferent arteriole

    Filtration depends on size and protein binding, not on urinary pH.

  • C. Salicylate becomes more ionised in tubular fluid, so it cannot be passively reabsorbed (correct)

    Correct. Ionised salicylate is polar and is trapped in alkaline urine.

  • D. Hepatic glucuronidation of salicylate is accelerated by the higher urinary pH

    Hepatic metabolism is unrelated to urinary pH, which acts only after the drug has been filtered.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, Salicylate becomes more ionised in tubular fluid, so it cannot be passively reabsorbed. Only the non-ionised fraction of a weak electrolyte crosses the tubular epithelium.

Why is "Salicylate becomes more lipid soluble, so it is secreted more activel…" not the answer?

Ionisation reduces, not increases, lipid solubility, and passive back-diffusion is what is being prevented.

Why is "Glomerular filtration of salicylate rises because alkalinisation dila…" not the answer?

Filtration depends on size and protein binding, not on urinary pH.

Why is "Hepatic glucuronidation of salicylate is accelerated by the higher ur…" not the answer?

Hepatic metabolism is unrelated to urinary pH, which acts only after the drug has been filtered.

What concept does this question test?

It tests Pharmacokinetics: ion trapping in Pharmacology. For a weak acid, raising the pH of tubular fluid above its pKa shifts it to the ionised form, which is trapped in the lumen and excreted. This ion trapping is the basis of urinary alkalinisation in salicylate and phenobarbitone poisoning.

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 Pharmacokinetics: ion trapping?

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.