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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.

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Biomolecules and enzymes MCQ (B72-BIOCHEMISTRY-5)

Q: What is the answer to this Biochemistry question?

The best answer is C, Potassium competes with digoxin for the binding site on the pump, so less potassium means more digoxin bound. Digoxin inhibits the Na+/K+ ATPase by binding the extracellular face of its alpha subunit, at a site that K+ also uses.

Q: What concept does this question test?

It tests Biomolecules and enzymes: digoxin and the sodium-potassium ATPase in Biochemistry. When extracellular K+ is low, digoxin binds more readily and inhibition is greater, raising intracellular Na+ and Ca2+ and causing toxicity. This is why potassium and magnesium are checked in digoxin-treated patients.

Q: How should I study Biomolecules and enzymes: digoxin and the sodium-potassium ATPase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

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Biochemistry · Biomolecules and enzymes: digoxin and the sodium-potassium ATPase

Sample item

A man with heart failure on digoxin is started on a loop diuretic and develops nausea, visual disturbance and arrhythmia. His serum potassium is 2.8 mmol/L. How does hypokalaemia increase digoxin toxicity?

APotassium loss displaces digoxin from plasma proteins and raises the free fractionBLow potassium increases the number of sodium-potassium pumps in cardiac cellsCPotassium competes with digoxin for the binding site on the pump, so less potassium means more digoxin boundDHypokalaemia reduces renal excretion of digoxin by blocking filtration

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

The answer and the reasoning

C. Potassium competes with digoxin for the binding site on the pump, so less potassium means more digoxin bound

Digoxin inhibits the Na+/K+ ATPase by binding the extracellular face of its alpha subunit, at a site that K+ also uses. When extracellular K+ is low, digoxin binds more readily and inhibition is greater, raising intracellular Na+ and Ca2+ and causing toxicity. This is why potassium and magnesium are checked in digoxin-treated patients.

Option by option

A. Potassium loss displaces digoxin from plasma proteins and raises the free fraction
Plasma protein binding of digoxin is low and is not the mechanism of its toxicity in hypokalaemia.
B. Low potassium increases the number of sodium-potassium pumps in cardiac cells
Hypokalaemia does not increase the number of pumps; digoxin toxicity arises from stronger inhibition of the existing pumps.
C. Potassium competes with digoxin for the binding site on the pump, so less potassium means more digoxin bound (correct)
Correct. K+ and digoxin compete for the same site on the alpha subunit, so hypokalaemia favours digoxin binding.
D. Hypokalaemia reduces renal excretion of digoxin by blocking filtration
Digoxin is cleared mainly by glomerular filtration and tubular secretion, and low potassium does not block filtration.

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.

Biomolecules and enzymes: digoxin and the sodium-potassium ATPase: practice question

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The answer and the reasoning

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