A 10-year-old boy presents with irritability and ataxia. He is subsequently found to have anemia, basophilic stippling of erythrocytes, and dark-gray pigmentation of the gums. Exposure to which of the following chemical agents is most likely associated with this disease?

Chronic lead poisoning inhibits delta-aminolevulinic acid dehydratase and ferrochelatase (enzymes essential for heme synthesis), thereby causing microcytic hypochromic anemia. The inhibition of heme synthesis leads to basophilic stippling of erythrocytes, which is due to residual ribosome clusters in the cytoplasm. Chronic exposure of children to lead also leads to cognitive loss. Mercury (choice D) poisoning has neurologic sequelae, but not these hematologic characteristics.Diagnosis: Lead poisoning