Medical Definition of mucous patch
: a broad, flat, erosive lesion of secondary syphilis that occurs on moist skin or mucous membranes and is often marked by a yellowish discharge During the secondary stage of syphilis, mucous patches, which have high concentrations of Treponema pallidum and are extremely infectious, might develop in the mouth.—The Journal of the American Medical Association, 24 Nov. 2004
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