A 3-year-old boy from a house undergoing renovation has pica, recurrent colicky abdominal pain, constipation, and a microcytic anaemia with marked basophilic stippling on the peripheral smear. Blood lead level is 55 microgram/dL. Lead impairs haem synthesis by directly inhibiting which two enzymes?

• A ALA synthase and porphobilinogen deaminase
• B ALA dehydratase and ferrochelatase ✓
• C Uroporphyrinogen decarboxylase and coproporphyrinogen oxidase
• D Protoporphyrinogen oxidase and ferrochelatase

Explanation

Lead inhibits ALA dehydratase, causing accumulation of delta-aminolevulinic acid, and ferrochelatase, blocking incorporation of iron into protoporphyrin IX. The latter produces raised free erythrocyte protoporphyrin and zinc protoporphyrin. Porphyrobilinogen deaminase is not inhibited, so porphobilinogen does not accumulate, which distinguishes lead poisoning biochemically from acute intermittent porphyria despite similar abdominal and neurological features.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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