From The Collaborative International Dictionary of English v.0.48:
Uroerythrin \U`ro*e*ryth"rin\, n. [See 1st Uro-, and
Erythrin.] (Physiol. Chem.)
A reddish urinary pigment, considered as the substance which
gives to the urine of rheumatism its characteristic color. It
also causes the red color often seen in deposits of urates.