A 48-year-old man complains of weakness and easy fatigability for 6 weeks. He has worked for 20 years in a chemical factory that produces a variety of plastics and other synthetic compounds. A complete blood count shows a hemoglobin level of 8.2 g/dL, WBC count of 45,000/mL, and a platelet count of 40,000/mL. Examination of a bone marrow aspirate reveals numerous malignant myeloblasts, and a diagnosis of acute myeloid leukemia is made. Exposure to which of the following agents is the most likely cause of this patient's hematologic disease?