Cardiovascular drugs: digoxin and potassium: 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 · Cardiovascular drugs: digoxin and potassium

Sample item

A 70-year-old man with heart failure on digoxin and furosemide has nausea and frequent ventricular ectopics. Serum digoxin is 1.5 ng/mL and potassium is 2.9 mmol/L. How does hypokalaemia precipitate toxicity at this level?

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

The answer and the reasoning

A. Potassium competes with digoxin for its binding site on Na+/K+ ATPase, so low potassium increases binding

Digoxin binds the extracellular potassium site of the sodium-potassium ATPase, and potassium competes for that site. In hypokalaemia more enzyme is inhibited, so toxicity occurs at plasma levels that are usually tolerated. Hypomagnesaemia and hypercalcaemia also sensitise the heart, and potassium should be corrected in patients on loop diuretics.

Option by option

  • A. Potassium competes with digoxin for its binding site on Na+/K+ ATPase, so low potassium increases binding (correct)

    Correct. Reduced competition from potassium increases pump inhibition.

  • B. Hypokalaemia increases hepatic metabolism to a toxic metabolite

    Digoxin is mostly excreted unchanged by the kidney and has no toxic hepatic metabolite.

  • C. Hypokalaemia reduces the volume of distribution of digoxin

    Digoxin binds tissue Na+/K+ ATPase widely and low potassium does not shrink its distribution.

  • D. Hypokalaemia induces P-glycoprotein and so reduces renal clearance

    P-glycoprotein induction would lower, not raise, digoxin levels, and is not linked to hypokalaemia.

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, Potassium competes with digoxin for its binding site on Na+/K+ ATPase, so low potassium increases binding. Digoxin binds the extracellular potassium site of the sodium-potassium ATPase, and potassium competes for that site.

Why is "Hypokalaemia increases hepatic metabolism to a toxic metabolite" not the answer?

Digoxin is mostly excreted unchanged by the kidney and has no toxic hepatic metabolite.

Why is "Hypokalaemia reduces the volume of distribution of digoxin" not the answer?

Digoxin binds tissue Na+/K+ ATPase widely and low potassium does not shrink its distribution.

Why is "Hypokalaemia induces P-glycoprotein and so reduces renal clearance" not the answer?

P-glycoprotein induction would lower, not raise, digoxin levels, and is not linked to hypokalaemia.

What concept does this question test?

It tests Cardiovascular drugs: digoxin and potassium in Pharmacology. In hypokalaemia more enzyme is inhibited, so toxicity occurs at plasma levels that are usually tolerated. Hypomagnesaemia and hypercalcaemia also sensitise the heart, and potassium should be corrected in patients on loop diuretics.

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 Cardiovascular drugs: digoxin and potassium?

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.