Derm Dx: Dark areas on a teenager's trunk

A 14-year-old moderately obese Hispanic female is concerned about dark areas on her trunk. These are asymptomatic but unsightly. Duration is approximately 6 months. Laboratory work-up, including thyroid-stimulating hormone and fasting glucose, revealed no abnormality. Topical ketoconazole proved ineffectual. On examination, well-demarcated hyperpigmented patches, many with slightly raised papules, are noted on her chest and back.