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indigo carmine

[ĭn′dĭ-gō′]
n.
  1. Sodium indigotindisulfonate; a blue dye used for measurement of kidney function and as a stain for Negri bodies.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.