Porphyria cutanea tarda is primarily caused by uroporphyrinogen decarboxylase deficiency (UROD).
The dermatological symptoms of PCT that include blistering and lesions on sun-exposed areas of the skin are caused by a buildup of porphyrin compounds (specifically uroporphyrinogen) close to the surface of the skin that have been oxidized by free radicals or sunlight [REF].