A patient on digoxin, potassium chloride clarithromycin, atenolol, and triamterene developed atrial fibrillation with controlled ventricular rate. Serum digoxin levels were raised. Which of his medications is responsible for the same?

Correct Answer: D. Clarithromycin

Clarithromycin is a macrolide antibiotic that is a potent inhibitor of the cytochrome P450 enzyme system, particularly CYP3A4. Digoxin is metabolized primarily by CYP3A4 in the liver and undergoes significant hepatic metabolism. When clarithromycin inhibits CYP3A4, it reduces the hepatic clearance of digoxin, leading to accumulation of the drug in the bloodstream and elevated serum digoxin levels. Additionally, clarithromycin can inhibit P-glycoprotein, a transporter protein responsible for digoxin efflux from enterocytes and renal tubules, further reducing digoxin elimination. The combination results in supratherapeutic digoxin levels, which increase the risk of digoxin toxicity. In this case, the elevated serum digoxin levels combined with new-onset atrial fibrillation (a known manifestation of digoxin toxicity) point directly to clarithromycin as the culprit. This is a classic drug–drug interaction seen in Indian clinical practice, particularly when macrolides are prescribed for respiratory or gastrointestinal infections in patients already on cardiac glycosides.

Why the other options are wrong

A. Atenolol — Atenolol is a beta-blocker that does not inhibit CYP450 enzymes or affect digoxin metabolism. While beta-blockers can slow AV nodal conduction and may contribute to rate control in atrial fibrillation, they do not increase serum digoxin levels. Atenolol is actually often used with digoxin for rate control in AF without causing digoxin accumulation. This is an NBE trap using a cardiac medication to distract from the true culprit. B. Triamterene — Triamterene is a potassium-sparing diuretic that does not inhibit CYP3A4 or interfere with digoxin metabolism. While potassium-sparing agents can increase serum potassium (which may increase digoxin toxicity risk), triamterene itself does not elevate digoxin levels. The question specifically states 'serum digoxin levels were raised,' not potassium levels, making triamterene an incorrect choice despite being a diuretic in the regimen. C. Potassium chloride — Potassium chloride supplementation does not inhibit digoxin metabolism or clearance. While hyperkalemia can potentiate digoxin toxicity by altering cardiac electrophysiology, potassium chloride does not cause elevated serum digoxin levels. The question explicitly states digoxin levels are raised, not that toxicity is due to electrolyte imbalance alone. This option confuses digoxin toxicity (which can be worsened by K+) with digoxin accumulation (caused by reduced clearance).

High-Yield Facts

Mnemonics

MACROLIDE CYP3A4 INHIBITORS Erythromycin, Clarithromycin (strong inhibitors); Azithromycin (weak/minimal). Remember: ECA — Erythromycin and Clarithromycin are the culprits; Azithromycin is safer. DIGOXIN TOXICITY SIGNS (DIG TOXIC) Dysrhythmias (PACs, AF, VT), Increased automaticity, GI upset, Toxicity (visual), Over-digitalization, Xanthopsia (yellow vision), Interference with AV node, Conduction blocks.

NBE Trap

NBE pairs a cardiac medication (atenolol) and a diuretic (triamterene) with digoxin to distract from the true culprit—the antibiotic. Students may reflexively choose a cardiac drug or confuse potassium-related toxicity with digoxin accumulation, missing the CYP3A4 inhibition mechanism.

Clinical Pearl

In Indian outpatient practice, macrolide antibiotics (especially clarithromycin) are frequently prescribed for respiratory infections in elderly patients already on digoxin for AF rate control. Always check for CYP3A4 inhibitors before prescribing macrolides; azithromycin is the safer alternative. Serum digoxin monitoring is essential when adding any CYP3A4 inhibitor.

_Reference: KD Tripathi Pharmacology Ch. 18 (Cardiac Glycosides & Drug Interactions); Harrison Ch. 226 (Atrial Fibrillation & Digoxin Toxicity)_