A 17-year-old adolescent girl noted a 2-cm annular pink, scaly lesion on her back. Over the next 2 weeks she develops several smaller oval pink lesions with a fine collarette of scale. They seem to run in the body folds and mainly involve the trunk, although a few occur on the upper arms and thighs. There is no lymphadenopathy and no oral lesions.
Which of the following is the most likely diagnosis?
A. Tinea versicolorThe description of this papulosquamous disease is classic for pityriasis rosea. This disease occurs in about 10% of the population, usually in young adults. Pityriasis rosea primarily affects the trunk and proximal extremities. Pityriasis rosea is usually asymptomatic, although some patients have an early, mild viral prodrome (malaise and low-grade fever), and itching may be significant. Drug eruptions, fungal infections, and secondary syphilis may mimic this disease. Fungal infections (tinea) are rarely as widespread and sudden in onset; potassium hydroxide (KOH) preparation will be positive. Psoriasis, with its thick, scaly plaques on extensor surfaces, should not cause confusion. A rare condition called guttate parapsoriasis should be suspected if the rash lasts more than 2 months, since pityriasis rosea usually clears spontaneously in 6 weeks. Lichen planus is a papulosquamous disorder, but it causes intensely pruritic polygonal plaques, often with intraoral involvement. It would not cause a “Christmas tree” pattern on the back as seen in this patient. Secondary syphilis is characterized by lymph-adenopathy, oral patches, and lesions on the palms and soles (a VDRL test will be strongly positive at this stage).