Cardiovascular Drugs USMLE Step 1 Practice Question
A 65-year-old man with a history of atrial fibrillation and normal renal function is prescribed digoxin 0.25 mg daily for rate control after inadequate response to metoprolol. His serum digoxin level three days ago was 1.5 ng/mL (therapeutic range 0.5-2.0 ng/mL) and serum potassium was 3.8 mEq/L. Hydrochlorothiazide 25 mg daily is then initiated for newly diagnosed hypertension. He presents three days later with nausea, vomiting, and yellow-green visual disturbances. Vital signs show blood pressure 138/86 mmHg, heart rate 140 bpm with regular rhythm, and respiratory rate 16/min. Physical examination is otherwise unremarkable. An ECG demonstrates a regular ventricular rate of 140 bpm with AV dissociation and no P waves visible. Laboratory studies reveal serum potassium 2.9 mEq/L, serum creatinine 0.9 mg/dL, and serum magnesium 1.6 mg/dL (normal 1.8-2.4 mg/dL). Which of the following best explains his clinical presentation?
Answer choices
- AAcute coronary syndrome with secondary arrhythmia and demand ischemia from tachycardia
- BProgression of atrial fibrillation to atrial flutter with 1:1 atrioventricular conduction
- CDigoxin toxicity secondary to thiazide-induced hypokalemia and hypomagnesemia increasing myocardial sensitivityCorrect answer
- DDigoxin-induced second-degree atrioventricular block with accelerated junctional escape rhythm
- EThiazide diuretic-induced hypokalemia causing increased automaticity without digoxin toxicity
- FHypomagnesemia-induced enhanced digitalis binding to cardiac Na-K-ATPase receptors
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