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Medical Term:

Congo red

Pronunciation: kong′gō red

Definition: An acid direct cotton dye, it is absorbed by amyloid and induces green fluorescence to amyloid in polarized light; used as a laboratory aid in the diagnosis of amyloidosis, as a histologic stain, and as an indicator (pH 3, blue-violet, to pH 5, red) in testing for free hydrochloric acid in gastric contents.

See: Bennhold Congo red stain