A 4-year-old boy living in an old house with peeling paint has abdominal pain, constipation, and Hb 8.5 g/dL with MCV 72 fL. Peripheral smear shows coarse basophilic stippling. Which two enzymes of heme synthesis are inhibited by the toxin responsible?
- A ALA synthase and porphobilinogen deaminase
- B Uroporphyrinogen decarboxylase and coproporphyrinogen oxidase
- C Ferrochelatase and ALA dehydratase ✓
- D Ferrochelatase and uroporphyrinogen synthase
Correct answer: C. Ferrochelatase and ALA dehydratase
Explanation
Lead inhibits ferrochelatase (iron incorporation into protoporphyrin) and ALA dehydratase (ALA to porphobilinogen), producing elevated free erythrocyte protoporphyrin and urinary ALA, with basophilic stippling from ribosomal RNA aggregation. Option A enzymes are not lead targets. Option B enzymes are affected in porphyria cutanea tarda and variegate porphyria respectively. Children with pica in lead-painted environments are the classic demographic for this exam scenario.
Reference: Nelson Textbook of Pediatrics, 22nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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