Congenital erythropoietic porphyria (CEP, Gunther's disease) presents with severe photosensitivity, haemolytic anaemia, and reddish-brown urine in infancy. The enzyme deficient in CEP and the accumulating porphyrin are:

• A Uroporphyrinogen III synthase; massive accumulation of uroporphyrin I and coproporphyrin I ✓
• B Uroporphyrinogen decarboxylase; accumulation of uroporphyrinogen III
• C Hydroxymethylbilane synthase; accumulation of porphobilinogen
• D Protoporphyrinogen oxidase; accumulation of protoporphyrin IX

Explanation

Uroporphyrinogen III synthase normally converts the linear tetrapyrrole HMB (hydroxymethylbilane) into the physiologically useful uroporphyrinogen III. Without this enzyme, HMB spontaneously cyclises to form the symmetric (non-physiological) uroporphyrinogen I, which is converted to coproporphyrin I. These type I isomers accumulate in erythrocytes, teeth (erythrodontia — pink-red teeth that fluoresce with Wood's lamp), skin (causing severe photomutilation), and urine. CEP is the most disfiguring porphyria. Bone marrow transplant is curative. This is distinct from PCT (porphyria cutanea tarda) which involves uroporphyrinogen decarboxylase.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.