A 7-year-old boy from an industrial area presents with irritability, vomiting, and abdominal colic. Examination reveals microcytic hypochromic anemia, basophilic stippling on blood smear, and Burton lines on the gingiva. Serum lead level is 65 µg/dL. The enzyme marked **A** in the heme synthesis pathway diagram is inhibited, along with ferrochelatase, leading to accumulation of ALA and zinc protoporphyrin. Which of the following is the most appropriate chelating agent for this child?