Lead Poisoning

Lead poisoning is a medium-yield topic on USMLE Step 1 that shows up in hematology questions about microcytic anemia and in toxicology vignettes about environmental or occupational exposures. The most common management error: defaulting to succimer for every case because it's the oral chelator. But once encephalopathy is present, you need BAL first — it crosses the blood-brain barrier — followed by EDTA. Succimer alone in an encephalopathic patient is inadequate and potentially dangerous. The core mechanism is inhibition of two heme synthesis enzymes simultaneously, causing basophilic stippling on smear and a sideroblastic-pattern anemia. The exam tests enzyme targets, age-specific exposures, and chelator selection based on severity.

What makes this topic tricky is that students tend to learn half of it. They know about basophilic stippling and maybe one enzyme, but miss that lead hits both ALA dehydratase and ferrochelatase — two distinct steps in heme synthesis. They also conflate pediatric and adult exposure patterns, which matters because vignettes are written around specific sources. The neurological features are another common trap: adult lead neuropathy causes wrist drop via radial nerve demyelination, not foot drop — and that distinction gets tested directly.

For USMLE Step 1, the management angle is where most points are lost. Students default to succimer for everything because it's the oral agent, but encephalopathic poisoning absolutely requires the BAL plus EDTA combination first. Knowing which chelator goes with which severity level — and being able to apply that to a vignette describing a symptomatic child versus an encephalopathic adult — is what separates a correct answer from a classic distractor pick.