We present the case of a 64-year-old male who presented to the emergency room because of recent onset palpitations. He was diagnosed with a first episode of atrial fibrillation and was managed at the emergency room by direct current cardioversion. The patient's past medical record was unremarkable, and no relevant findings were discovered during the physical examination or laboratory tests. The baseline ECG is shown in Figure 1 . On discharge the patient was referred for outpatient cardiology

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... aluation and was eventually seen 2 months later. The ECG at the outpatient clinic is shown in Figure 2 . Which drug was most likely prescribed to the patient to account for the differences between the first and second ECGs? Please turn the page to read the diagnosis.