A 16 year-old presents for evaluation of a rash. The patient notes that it started on their back, with multiple spots, and has spread to their upper chest and arms. It itches a little. The patient also reports sweating more than before because their new summer job involves being outdoors. On physical examination, you note hyperpigmented, oval macules scattered more prominently around the upper back, chest, neck, and upper arms, as well as under the arms. Based on this description, what is the most likely diagnosis?