Children 6 to 10 years old in the same community are observed by the local physician to be doing poorly in school, which has been attributed to behavioral problems. Their parents state that these children have poor appetites, complain of nausea, and have frequent headaches. On physical examination, they have decreased sensation to touch over the lower extremities. They exhibit loss of fine motor control of movement and have a slightly ataxic gait. A representative CBC shows hemoglobin of 11.8 g/dL, hematocrit of 35.2%, MCV of 82 mm3, platelet count of 282,300/ mm3, and WBC count of 4745/ mm3. Examination of the peripheral blood smear shows basophilic stippling of the RBCs. Excessive chronic ingestion of which of the following substances is most likely to explain these findings?