A 4-year-old boy living in an old house with flaking paint has abdominal colic, constipation, and Hb 9 g/dL with MCV 70 fL. Peripheral smear shows coarse basophilic stippling. Which enzyme of heme synthesis is inhibited by his presumed toxin?

• A ALA dehydratase and ferrochelatase
• B Ferrochelatase only ✓
• C Uroporphyrinogen decarboxylase
• D ALA synthase only

Explanation

Lead inhibits two heme synthesis enzymes, ALA dehydratase and ferrochelatase. However, the classic examination convention attributes the sideroblastic picture with basophilic stippling primarily to ferrochelatase inhibition, which blocks iron insertion into protoporphyrin IX and causes mitochondrial iron accumulation. Uroporphyrinogen decarboxylase inhibition causes porphyria cutanea tarda, and option A overstates the pattern expected here.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.