Porphyria

n. (Pathology) excess of porphyrins in the blood and urine (hereditary disorder)

Porphyrias are a group of inherited or acquired disorders of certain enzymes in the heme biosynthetic pathway (also called porphyrin pathway). They are broadly classified as hepatic porphyrias or erythropoietic porphyrias, based on the site of the overproduction and mainly accumulation of the porphyrins (or their chemical precursors). They manifest with either skin problems or with neurological complications (or occasionally both).
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Porphyria /por·phy·ria/ any of a group of disturbances of porphyrin metabolism characterized by increase in formation and excretion of porphyrins or [more]Porphyria - Community and Resources

Noun
1. a genetic abnormality of metabolism causing abdominal pains and mental confusion
(hypernym) genetic disease, genetic disorder, genetic abnormality, genetic defect, congenital disease, inherited disease, inherited disorder, hereditary disease, hereditary condition

A varied series of hereditary diseases with increased formation and excretion of chemicals called porphyrins. One type of porphyria -- acute intermittent porphyria -- may have affected members of the House of Hanover in England including Mad King George who may not have been mad but suffering attacks of porphyria.