pityriasis rosea

pityriasis ro·sea -'rō-zē-ə n an acute benign and self-limited skin eruption of unknown cause that consists of dry, scaly, oval, pinkish or fawn-colored papules, usu. lasts six to eight weeks, and affects esp. the trunk, arms, and thighs

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a common, acute or subacute, self-limited, exanthematous disease of unknown etiology, whose onset is marked by a solitary pink, reddish, or tan plaque called a herald patch, usually on the trunk, upper limbs, or thighs; subsequent lesions are similar to this but smaller, with vesicular borders, tending to peel and produce a scaly collarette.

Herald patch and surrounding lesions of pityriasis rosea.