Pityriasis rosea is a benign, common, self-limited (goes away on its own) rash of older children and young adults. It often (but not always) begins with a so-called "herald patch," which is a red, slightly scaly 0.5-1 inch oval area of red skin on your torso; this is soon followed by many similar but smaller lesions. Thankfully, those afflicted are generally otherwise well, though they may complain of mild itching. In typical cases, the long axis of the oval lesions is oriented in a characteristic "V" pattern on your back and an "M" pattern in front, sparing your face and extremities. To some holiday-oriented physicians, the back appears to have lesions in a Christmas tree pattern. The scaling of each individual ½ inch red lesion often forms as a "collarette" on the inside edge. The disease typically lasts about four to six weeks and is unlikely to recur. During the first few weeks, it reaches its maximum extent, followed by a stable phae lasting a few weeks, after which it resolves, so you must be patient and let the condition run its course.
Those with darker skin are likely to have a more exuberant (sometimes with small vesicles instead of scaling) and itchy eruption. Since there is no cure and the condition spontaneously remits, the most important factor is an accurate diagnosis in order to distinguish this condition from other treatable diseases. Traditionally, the appearance of secondary syphilis is often confused with pityriasis rosea so a simple blood test is used to differentiate the two. A skin biopsy occasionally can also lend support to the diagnosis of a variety of other clinically similar problems.
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Chuh, A., H. Chan, and V. Zawar. "Pityriasis rosea -- evidence for and against an infectious aetiology." Epidemiol Infect 132.3 June 2004: 381-390.