Is this skin diseased, or is it just dirty?

CASE

Two brothers, aged 9 and 7 years, were brought to our clinic by their mother. The boys had an unaesthetic dirty appearance to the skin on their trunks, arms, and legs of some months' duration. Physical examination of both boys disclosed brown-to-dark, velvety textured, hyperpigmented areas on the abdomen and umbilical region, with light scaling, and on the dorsal and volar aspects of the upper and lower limbs (Figure 1). The neck and dorsum were not involved.

The mother assured us that the boys were otherwise healthy, were not taking any medications or nutritional supplements, and maintained good hygiene, including daily showers with shampoos and soaps. She denied her consent to perform dermal scraping or punch biopsy on any lesional area. Skin examination with Wood's light disclosed no fluorescence.