A nurse is collecting data from a client who reports a sudden onset of shortness of breath, fatigue, and dizziness. Electrocardiography indicates the client is experiencing atrial fibrillation. Which of the following medications should the nurse expect to administer?

A. Vitamin K: Vitamin K reverses warfarin anticoagulation; it is not used to treat atrial fibrillation acutely or to control rate. B. Digoxin: Correct (best choice among options). Digoxin slows AV nodal conduction and can help control ventricular rate in atrial fibrillation, especially when other agents are contraindicated or in heart failure. (Note: in many acute settings, beta-blockers or nondihydropyridine calcium channel blockers are commonly used for rapid rate control, but those were not offered.) C. Atropine: Atropine increases heart rate and is used for symptomatic bradycardia; it is not indicated for atrial fibrillation with rapid ventricular response. D. Magnesium: Magnesium is used for certain arrhythmias (e.g., torsades de pointes) and electrolyte correction, not first-line therapy for atrial fibrillation.