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The porphyrias are metabolic disorders, each resulting from the deficiency or increased activity of a specific enzyme in the heme biosynthetic pathway (Fig. 409-1 and Table 409-1). These enzyme disorders are inherited as autosomal dominant, autosomal recessive, or X-linked traits, with the exception of porphyria cutanea tarda (PCT), which is usually sporadic (Table 409-1). The porphyrias are classified as either hepatic or erythropoietic, depending on the primary site of overproduction and accumulation of their respective porphyrin precursors or porphyrins (Tables 409-1 and 409-2), although some have overlapping features. For example, PCT, the most common porphyria, is hepatic and presents with blistering cutaneous photosensitivity, which is typically characteristic of the erythropoietic porphyrias (EPPs).

FIGURE 409-1

The human heme biosynthetic pathway indicating in linked boxes the enzyme that, when deficient or overexpressed, causes the respective porphyria. Hepatic porphyrias are shown in yellow boxes and erythropoietic porphyrias in pink boxes.

TABLE 409-1Human Porphyrias: Major Clinical and Laboratory Features
TABLE 409-2Human HEME Biosynthetic Enzymes and Genes

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