A 72-year-old man with a history of heart failure and hypertension is admitted to the hospital with pneumonia. His medications include furosemide, ramipril, and digoxin. On day 3 of admission, he develops new-onset polymorphic ventricular tachycardia (Torsades de Pointes). An ECG shows a QTc interval of 580 ms. Which of the following is the MOST likely contributing factor to his arrhythmia?