Unilateral pityriasis rosea.

PR commonly presents on the trunk as oval scaly papules and plaques arranged along skin tension lines in a "Christmas tree" pattern.[1] In its typical form, PR is easily recognizable clinically. The strictly unilateral involvement is rarely seen in PR.[2] In our patient, the diagnosis of PR was made based on the acute onset of lesions, their distibution along skin tension lines, and the presence of a herald patch. Even though histological findings are not specific in PR, lesional skin biopsy may be mandatory in atypical forms so that important differential diagnoses, such as drug eruption, secondary syphilis, or psoriasis, are not missed. [Extracted from the article]