Antibacterials: trimethoprim and hyperkalaemia: 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 · Antibacterials: trimethoprim and hyperkalaemia

Sample item

A 35-year-old man with HIV receiving high-dose co-trimoxazole for Pneumocystis pneumonia has a potassium of 5.8 mmol/L and a creatinine rise from 0.9 to 1.3 mg/dL, with normal urine output and a bland urine sediment. What best explains the hyperkalaemia?

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

The answer and the reasoning

A. Trimethoprim blocks epithelial sodium channels in the collecting duct, like amiloride

Trimethoprim is structurally related to amiloride and blocks collecting duct ENaC, reducing potassium secretion, particularly at high doses and with renal impairment or ACE inhibitors. It also competes for tubular creatinine secretion, raising serum creatinine without a true fall in filtration. The sediment is bland, which makes crystal nephropathy less likely.

Option by option

  • A. Trimethoprim blocks epithelial sodium channels in the collecting duct, like amiloride (correct)

    Correct. Amiloride-like ENaC block reduces potassium excretion.

  • B. Sulfamethoxazole causes adrenal insufficiency

    Sulfamethoxazole does not cause adrenal insufficiency.

  • C. Sulfamethoxazole crystals obstruct the distal nephron

    Crystal nephropathy would be expected to show crystals and reduced urine output, which is not seen here.

  • D. Trimethoprim inhibits the Na-K-ATPase in the proximal tubule

    Trimethoprim does not inhibit the proximal Na-K-ATPase; the target is the collecting duct ENaC.

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 A, Trimethoprim blocks epithelial sodium channels in the collecting duct, like amiloride. Trimethoprim is structurally related to amiloride and blocks collecting duct ENaC, reducing potassium secretion, particularly at high doses and with renal impairment or ACE inhibitors.

Why is "Sulfamethoxazole causes adrenal insufficiency" not the answer?

Sulfamethoxazole does not cause adrenal insufficiency.

Why is "Sulfamethoxazole crystals obstruct the distal nephron" not the answer?

Crystal nephropathy would be expected to show crystals and reduced urine output, which is not seen here.

Why is "Trimethoprim inhibits the Na-K-ATPase in the proximal tubule" not the answer?

Trimethoprim does not inhibit the proximal Na-K-ATPase; the target is the collecting duct ENaC.

What concept does this question test?

It tests Antibacterials: trimethoprim and hyperkalaemia in Pharmacology. It also competes for tubular creatinine secretion, raising serum creatinine without a true fall in filtration. The sediment is bland, which makes crystal nephropathy less likely.

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 Antibacterials: trimethoprim and hyperkalaemia?

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.