A patient stabilized on digoxin for atrial fibrillation is given quinidine for rhythm control. Within days she develops nausea, visual disturbance, and ECG changes consistent with digitalis excess. The most likely explanation is:

• A Quinidine induces hepatic CYP3A4, accelerating digoxin clearance
• B Quinidine causes hypokalemia, sensitizing the myocardium to digoxin
• C Quinidine antagonizes digoxin at Na+/K+-ATPase, requiring higher digoxin doses
• D Quinidine displaces digoxin from skeletal muscle binding sites and reduces its renal clearance, raising serum digoxin concentration ✓

Explanation

Quinidine reduces the volume of distribution of digoxin by displacing it from tissue binding sites (especially skeletal muscle) and decreases both renal and non-renal digoxin clearance, roughly doubling serum digoxin concentrations. The digoxin dose should be halved when quinidine is initiated. Option A is the opposite of what happens, since quinidine inhibits rather than induces P-glycoprotein-mediated digoxin transport. Quinidine tends to cause hypokalemia indirectly, but the dominant recognized interaction is the pharmacokinetic displacement and clearance reduction.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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