The goal of this clinical research study is to compare the effectiveness of receiving a combination of ondansetron and aprepitant to receiving ondansetron alone in helping to prevent nausea and/or vomiting in patients with Acute myeloid leukemia (AML) or high-risk (HR) Myelodysplastic syndromes (MDS) who are receiving cytarabine. The safety of this drug combination will also be studied.