In congenital erythropoietic porphyria, deficiency of uroporphyrinogen III synthase means that hydroxymethylbilane spontaneously cyclises into which abnormal product?

• A Delta-aminolevulinic acid dimers
• B Protoporphyrin IX lacking iron
• C Uroporphyrinogen I, leading to coproporphyrinogen I accumulation ✓
• D Zinc protoporphyrin within erythrocytes

Explanation

Uroporphyrinogen III synthase normally flips over hydroxymethylbilane to produce the asymmetric type III isomer required for heme synthesis. Without it, spontaneous cyclisation yields the symmetric type I isomers: uroporphyrinogen I, then coproporphyrinogen I, which accumulate in tissues, teeth, urine, and bone marrow and fluoresce intensely red. These type I porphyrins cannot proceed further down the pathway. Zinc protoporphyrin instead marks iron deficiency or lead poisoning.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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