Congenital erythropoietic porphyria (Gunther disease) is caused by deficiency of uroporphyrinogen III synthase. Which uroporphyrin isomer characteristically accumulates?

• A Type I uroporphyrin only ✓
• B Type III uroporphyrin only
• C Both type I and type III in equal amounts
• D Uroporphyrinogen does not accumulate; coproporphyrin does

Explanation

Uroporphyrinogen III synthase converts hydroxymethylbilane to uroporphyrinogen III. When deficient, the non-enzymatic pathway produces the type I isomer, which cannot be used for heme synthesis. Type I uroporphyrin accumulates in bone marrow, teeth (erythrodontia), and urine. Type III is the physiologically useful isomer.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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