Several children between the ages of 3 and 6 years have been admitted to a local hospital because of the encephalopathic crisis. They have lived in the same community all their lives. All have previously exhibited retarded psychomotor development. On physical examination, the children have diffuse abdominal pain and are experiencing nausea and vomiting. Head CT scans show marked cerebral edema. Laboratory studies show microcytic anemia. An investigator sent to the housing project where the children live finds a rundown apartment complex with extensive water damage, poor plumbing, and ventilation. Toxic exposure to which of the following substances best accounts for these findings?