Image of the Week: What's causing this greasy underarm rash?

A 17-year-old male presents with a one-year history of a recurrent pruritic rash on his back, shoulders, hands, and axillae. He mentions that the rash is typically worse in the summer months and similar in appearance to one his mother has. Examination reveals numerous brown keratotic papules with a greasy texture and multiple cobblestone lesions in the mouth. A skin ...