What is the likely etiology of darkened skin and light-colored lesions on the trunk after sun exposure?

The combination of darkened skin and light-colored lesions on the trunk after sun exposure suggests a condition that is typically associated with a change in skin pigmentation due to factors such as sun exposure. Tinea Versicolor, a fungal skin infection caused by the overgrowth of Malassezia yeast, often leads to light or hypopigmented patches on a darker skin background. This is particularly noticeable after sun exposure because the affected areas do not tan like the surrounding skin, resulting in a contrast that can look like light-colored lesions against a darker backdrop.

In contrast, psoriasis typically presents with raised, red plaques covered with silvery scales, which usually do not manifest as light-colored lesions following sun exposure. Actinic keratosis is associated with rough, scaly patches that occur due to sun damage but do not typically appear as light-colored lesions against darkened skin. Melasma is characterized by brown or tan patches, particularly on sun-exposed areas, but does not lead to light-colored lesions.

This distinctive presentation of Tinea Versicolor aligns perfectly with the observed changes after sun exposure, which makes it the most plausible etiology in this scenario.