<s>[INST] context: (b) Source: (Hay et al, p 397. Kliegman et al, p 2262. McMillan et al, pp 839-840. Rudolph et al, p 1262.) Pityriasis rosea is a benign condition that usually presents with a herald patch, a single round or oval lesion appearing anywhere on the body. Usually about 5 to 10 days after the appearance of the herald patch, a more diffuse rash involving the upper extremities and trunk appears. These lesions are oval or round, slightly raised, and pink to brown in color. The lesion is covered in a fine scale with some central clearing possible. The rash can appear in a Christmas tree pattern on the back, identified by the aligning of the long axis of the lesions with the cutaneous cleavage lines. The rash lasts 2 to 12 weeks and can be pruritic. This rash is commonly mistaken for tinea corporis, and the consideration of secondary syphilis is important.Treatment is usually unnecessary but can consist of topical emollients and oral antihistamines, as needed. More uncommonly, topical steroids can be helpful if the itching is severe. Lichen planus is rare in children. It is intensely pruritic, and additional lesions can be induced with scratching. The lesion is commonly found on the flexor surfaces of the wrists, forearms, inner thighs, and occasionally on the oral mucosa. Seborrheic dermatitis can begin anytime during life; it frequently presents as cradle cap in the newborn period. This rash is commonly greasy, scaly, and erythematous and, in smaller children, involves the face, neck, axilla, and diaper area. In older children, the rash can be localized to the scalp and intertriginous areas. Pruritus can be marked. Contact dermatitis is characterized by redness, weeping, and oozing of the affected skin. The pattern of distribution can be helpful in identification of the offending agent. The rash can be pruritic; removal of the causative agent and use of topical emollients or steroids is curative. Psoriasis consists of red papules that coalesce to form plaques with sharp edges. A thick, silvery scale develops on the surface and leaves a drop of blood upon its removal (Auspitz sign). Additional lesions develop upon scratching older lesions. Commonly affected sites include scalp, knees, elbows, umbilicus, and genitalia. [/INST] question: The adolescent shown presents with a 14-day history of multiple oval lesions over his back. The rash began with a single lesion over the lower abdomen (A); the other lesions developed over the next days (B). These lesions are slightly pruritic. Which of the following is the most likely diagnosis? | options: A: Contact dermatitis B: Pityriasis rosea C: Seborrheic dermatitis D: Lichen planus | answer: B </s>