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

Lead poisoning gives a pale, small celled anemia that looks like iron deficiency on a blood film, but the real problem is not a lack of iron. It is a broken assembly line for heme.

1. Inhibition of enzymes involved in heme biosynthesis: Lead has a strong attraction for the sulfhydryl groups on two heme pathway enzymes, ALA dehydratase and ferrochelatase. Blocking them stalls heme production at two points, so hemoglobin content drops in each red cell even though iron itself is available. This directly explains the hypochromic picture and matches the raised free erythrocyte protoporphyrin seen in lead toxicity.
2. Binding of lead to transferrin, inhibiting transport of iron: This would stop iron reaching the marrow at all, giving a picture closer to iron deficiency from poor supply, but this is not how lead actually behaves; lead does not meaningfully block transferrin's iron carrying job.
3. Binding of lead to cell membrane of erythroid precursors: While lead can shorten red cell lifespan and add a mild hemolytic component, membrane damage is not the accepted main mechanism of the anemia's hypochromic nature.
4. Binding of lead to ferritin inhibiting their breakdown into hemosiderin: Ferritin to hemosiderin conversion is a storage-form change, not a step in making new hemoglobin, so blocking it would not explain reduced hemoglobin in circulating cells.

The pathway breaks at the enzyme level, so the correct explanation is inhibition of heme synthesis enzymes.

Let's summarize:

• Lead blocks ALA dehydratase (early step) and ferrochelatase (final step) of heme synthesis.
• Iron stores are usually normal or high in lead poisoning, unlike true iron deficiency.

The correct answer is inhibition of enzymes involved in heme biosynthesis.