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urobilinogen (yoor-oh-by-lin-ŏ-jin) n. a colourless product of the reduction of the bile pigment bilirubin. Urobilinogen is formed from bilirubin in the intestine by bacterial action. Part of it is reabsorbed and returned to the liver; part of it is excreted. When exposed to air, urobilinogen is oxidized to a brown pigment, urobilin.