An 68-year-old diabetic man on metformin has a hypochromic microcytic anemia unresponsive to oral iron. Bone marrow aspirate stained with Prussian blue shows nucleated red cell precursors with a perinuclear collar of granules. What does this finding represent?

• A Iron deposited within lysosomes of erythroid precursors
• B Residual DNA fragments in maturing erythroblasts
• C Iron accumulated in mitochondria surrounding the nucleus of erythroid precursors ✓
• D Aggregated ribosomes precipitated by the staining procedure

Explanation

Ring sideroblasts are erythroid precursors with at least five iron-loaded mitochondria arranged as a perinuclear collar on Prussian blue stain. The defect lies in mitochondrial heme synthesis, so iron enters the mitochondrion but cannot be inserted into protoporphyrin by ferrochelatase, and accumulates there. Metformin impairs C12 absorption, and acquired sideroblastic anemia is a recognized cause of refractory microcytic anemia. Lysosomal iron gives ordinary siderosomes, not rings, excluding option A.

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

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