Question

The pathogenesis of hypochromic anemia in lead poisoning is due to

• A. Inhibition of enzymes involved in heme biosynthesis
• B. Binding of lead to transferrin, inhibiting transport of iron
• C. Binding of lead to cell membrane of erythroid precursors
• D. Binding of lead to ferritin inhibiting their breakdown into hemosiderin

Hint:

Lead blocks ALA dehydratase and ferrochelatase in the heme synthesis pathway, so hemoglobin cannot be made properly despite normal iron stores.

Answer 1

The Correct Option is A

Step 1: Understanding the Question.
Lead poisoning causes a hypochromic anemia, meaning red cells are pale from low hemoglobin content, even though the marrow usually has enough iron. We need the exact mechanism behind this.

Step 2: Key Concept.
Heme is built through a multi step pathway inside developing red cells, and two of its key enzymes, delta-aminolevulinic acid dehydratase (ALA dehydratase) and ferrochelatase, are extremely sensitive to lead. Lead binds and blocks these sulfhydryl dependent enzymes, so the pathway cannot finish making heme even when iron is present.

Step 3: Detailed Explanation.
With ALA dehydratase blocked, delta-ALA piles up before the block and heme production slows down. With ferrochelatase blocked, the final step of inserting iron into protoporphyrin IX fails, so protoporphyrin accumulates in red cells (seen as raised free erythrocyte protoporphyrin, an important clue used in lead poisoning testing) while hemoglobin content falls.
The result is red cells that are small and pale (microcytic, hypochromic), often with basophilic stippling on the smear from residual ribosomal RNA, even though total body iron is not the problem.
Option (2) is incorrect because lead does not act by displacing iron off transferrin; the anemia comes from blocked heme synthesis, not blocked iron transport.
Option (3) is incorrect because lead's toxicity to red cell precursors is mainly through enzyme inhibition inside the synthetic pathway, not by damaging the outer cell membrane as the primary mechanism of this anemia.
Option (4) is incorrect because ferritin to hemosiderin conversion is not the step lead interferes with; the defect is earlier, in heme synthesis itself.

Step 4: Final Answer.
The hypochromic anemia of lead poisoning is due to inhibition of enzymes involved in heme biosynthesis, option (1).