A 32-year-old woman undergoing chemotherapy for breast cancer (doxorubicin + cyclophosphamide) presents to the oncology clinic for her second cycle. During the first cycle, she experienced severe nausea and vomiting despite receiving ondansetron 8 mg IV before treatment. Her oncologist decides to add a second antiemetic agent to improve control. Which of the following is the most appropriate addition based on mechanism of action?