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A 2yo child is brought in after having taken some of Grandma’s “heart medicine.” Because the pills are kept in an unlabeled box and not their original container, the parents don’t know what the medication was. The child has sinus bradycardia with frequent PVC’s on the ECG, a blood glucose of 90; electrolytes on the i-stat are normal except for a slightly elevated potassium level.

Which is the most likely drug?