A 72-year-old woman is admitted to the cardiology ward following an episode of syncope. Her past medical history includes hypertension, for which she takes hydrochlorothiazide, and osteoarthritis, managed with regular ibuprofen. An ECG reveals a prolonged QTc interval of 520 ms. She denies any family history of sudden cardiac death or prolonged QT syndrome. Her electrolytes are within normal limits except for a potassium level of 3.4 mmol/L (normal range 3.5-5.0 mmol/L). Which of the following is the MOST appropriate initial step in managing this patient?