Medical Definition of bilirubin
: a reddish-yellow, crystalline, water-insoluble pigment C33H36N4O6 that is formed by the breakdown of heme, is excreted in a water-soluble conjugated form by liver cells into bile, and occurs in blood and urine especially in diseased states (such as cirrhosis, gallstones, and hemolytic anemia) … they may become jaundiced, their skin tinged yellow by the release of the pigment bilirubin from their damaged liver.—Larry Thompson, Discover, March 1994
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