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WordNet
pityriasis rosea

n. pityriasis in which an itchy rash develops over the trunk and extremities

Wikipedia
Pityriasis rosea

Pityriasis rosea (also known as pityriasis rosea Gibert) is a skin rash. It is benign but may inflict substantial discomfort in certain cases. Classically, it begins with a single "herald patch" lesion, followed in 1 or 2 weeks by a generalized body rash lasting up to 12 weeks, however usually around 6 - 8.