A 56-year-old woman with metastatic breast cancer is receiving a doxorubicin-based chemotherapy regimen. Prior to each infusion, she is given an 8 mg dose of ondansetron intravenously to prevent chemotherapy-induced nausea and vomiting. During her second cycle, she reports palpitations shortly after the ondansetron infusion, and continuous telemetry reveals an increased QTc interval. A 12-lead ECG shows a QTc of 510 ms compared with a baseline of 420 ms. Which of the following mechanisms best explains this adverse effect of ondansetron?