Final answer:
The adverse effect of the medication is a prolonged QT interval, which can increase the risk of life-threatening arrhythmias.
Step-by-step explanation:
The nurse should identify a prolonged QT interval as an adverse effect of the medication. The QT interval represents the time from the start of depolarization to repolarization of the ventricles and includes the period of ventricular systole. Prolongation of the QT interval can increase the risk of life-threatening arrhythmias, such as torsades de pointes.