A 7-year-old boy is brought to the pediatric cardiology clinic after a near-drowning incident while swimming at school. He is otherwise healthy with no prior cardiac symptoms. His 12-lead ECG shows a **QTc interval of 520 ms** (marked **A** in the diagram). His parents report that his maternal uncle died suddenly at age 16 during a swimming competition. Genetic testing confirms a KCNQ1 mutation. Which of the following is the most appropriate immediate management strategy for this patient?