A 24-year-old woman comes to the clinic because of a new rash on her back and shoulders for 3 weeks. She recently started taking oral prednisone for an asthma exacerbation. She reports mild pruritus but no fever. Skin examination shows numerous 1–3 mm uniform erythematous papules and pustules distributed over the upper back, shoulders, and upper arms. The lesions appear to be in the same stage of development. No open or closed comedones are seen. Which of the following is the most likely diagnosis?