A woman with black urine

A 46-year-old woman presented at our clinic for a nephrologicopinion because of moderate renal failure. She was known tosuffer from acute intermittent porphyria, a rare inheritedmetabolic disorder caused by disturbed porphyrin metabolism,resulting in increased production of porphyrin or its precur-sors. Her medical history was notable for abdominal pain,photosensitivity and neurological disturbances, including sei-zures and hallucinations. Genetic analysis showed a missensemutation (242T>C) in exon 6 of the gene coding forhydroxymethylbilane synthase, the third enzyme in the haembiosynthetic pathway, which catalyses the stepwise condensa-tion of four monopyrrole porphobilinogen molecules to yieldthe linear tetrapyrrole hydroxymethylbilane. Her urine becameblack after exposure to light (Fig). Darkened urine because ofan increased excretion of porphyrins is often a crucial step thathelps in a not always easy diagnosis.Piero Stratta