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PhotoallergySometimes contact allergy arises only after the skin has been exposed to ultraviolet light.The rash is confined to sun exposed areas even though the allergen may have been in contact with covered areas.This is called photocontact dermatitis. Examples include:Dermatitis due to a sunscreen chemical, affecting the top but not the under surface of the arm
Figure 7. Palpable purpura can appear in many different patterns . This picture shows a denser distribution with a sharp demarcation caused by what is known as Koebner's Phenomenon (Minor trauma, such as the elastic band in one's sock, can cause such a pattern). In this case, the “trauma” was caused by the patient's shaving of her legs, leading to the eruption of purpura in the area of skin where the razor had passed.