A 28-year-old man with atypical pneumonia (suspected *Mycoplasma pneumoniae*) is started on azithromycin 500 mg once daily. On day 5, he develops palpitations, dizziness, and syncope. ECG shows a prolonged QTc interval of 520 ms (baseline 400 ms). Serum potassium is 3.2 mEq/L (normal 3.5–5.0). What is the most appropriate immediate next step?