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?

Explanation

## Why Succimer (DMSA) is right Succimer (meso-2,3-dimercaptosuccinic acid) is the first-line oral chelating agent for pediatric lead poisoning with blood lead levels 45–69 µg/dL. It is lipophilic, crosses the blood–brain barrier, and is well-tolerated in children. The clinical presentation—irritability, abdominal colic, basophilic stippling, Burton lines, and microcytic anemia—is classic for lead poisoning caused by inhibition of **A** (ALA synthase) and ferrochelatase, preventing heme synthesis and causing accumulation of ALA and zinc protoporphyrin. Succimer is the preferred oral option for this age group and severity level (Harper 32e Ch 31; Harrison 21e Ch 415). ## Why each distractor is wrong - **EDTA (ethylenediaminetetraacetic acid)**: Intravenous chelator reserved for severe lead poisoning (blood lead >70 µg/dL) or symptomatic encephalopathy. Not first-line for this moderate presentation in a child. - **Dimercaprol (BAL)**: Intramuscular agent used in combination with EDTA for severe toxicity or encephalopathy. Not monotherapy for moderate poisoning; also more toxic and less convenient than succimer. - **Penicillamine**: Used for copper toxicity (Wilson disease) and mercury poisoning, not lead. Slower onset and higher risk of adverse effects compared to succimer in lead poisoning. **High-Yield:** Succimer (DMSA) is the preferred first-line oral chelator for pediatric lead poisoning; EDTA is reserved for severe cases or encephalopathy. [cite: Harper 32e Ch 31; Harrison 21e Ch 415]