A 40-year-old woman with microcytic anemia shows ringed sideroblasts in the bone marrow. Iron studies reveal high serum iron and high ferritin. Pyridoxine (vitamin B6) deficiency impairs which enzyme, accounting for this pattern?

• A ALA dehydratase
• B Ferrochelatase
• C Delta-aminolevulinic acid synthase ✓
• D Coproporphyrinogen oxidase

Explanation

Delta-ALA synthase, the rate-limiting first enzyme of heme synthesis in mitochondria, is a pyridoxal phosphate-dependent enzyme condensing glycine and succinyl-CoA. Its impairment reduces heme production, so iron accumulates in mitochondria forming ringed sideroblasts. Ferrochelatase deficiency causes erythropoietic protoporphyria with protoporphyrin accumulation, and ALA dehydratase inhibition by lead causes accumulation of ALA, neither of which matches the sideroblastic picture.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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