A healthy 10-year-old male presents for evaluation with a 3-year history of an asymptomatic and progressive, mildly pruritic rash over his head and trunk. The first lesion appeared on his back 3 years ago, and numerous other lesions developed insidiously afterward. The patient’s father states that the lesions fade during the winter and become more prominent during the summer. Failed treatment included hydrocortisone. What's the diagnosis?