USMLE STEP 3 MCQ 48:A fifty-five year old with chronic cardiac failure





A fifty-five year old with chronic cardiac failure is on treatment with digoxin and a loop diuretic.
This combination is likely to cause digoxin toxicity by the following mechanism:


a) this combination increases the half-life of digoxin
b) diuretics decrease potassium levels
c) frusemide and digoxin interact to form a poisonous compound
d) digoxin is a competitive inhibitor of frusemide
e) the above statement is wrong, loop diuretics are not a recognised cause of increased digoxin toxicity

Correct Answer: B


Loop and thiazide diuretics decrease potassium and magnesium levels, predisposing patients taking both a diuretic and digoxin to an increased risk of digoxin toxicity. Also, amphotericin B (Fungizone), an antifungal, has an additive potassium-lowering effect when given with a thiazide or loop diuretic. Thiazides may increase the blood levels of lithium. Bile acid sequestrants cholestyramine (Questran) and colestipol (Colestid) decrease the absorption of thiazide diuretics when given concomitantly, while nonsteroidal anti-inflammatory drugs such as indomethacin (Indocin) may decrease the therapeutic effects of both the loop and thiazide diuretics.

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