A 10-year-old boy has had a lesion on his left foot for almost a year. It has not responded to either topical antifungal cream (econazole, applied twice daily for weeks) or, subsequently, topical corticosteroid cream (mometazone, also applied twice daily). Frustrated by the lack of resolution, his mother brings him for evaluation.
The condition began with faint linear scaling, the area of which became gradually wider and longer. The child reports no associated symptoms, and the mother denies seeing her son manipulate, rub, or scratch the affected skin.
Aside from mild atopy—in the form of seasonal allergies and asthma—the boy is healthy.
The child is well developed, well nourished, and in no distress. He gladly permits examination of the lesion, which is located on the dorsum of the left foot, running from the lower leg to just proximal to the toes. The linear strip of skin measures 2 cm at its widest point. The lesion is tan and uniformly scaly; it exhibits no overt signs of inflammation or increased warmth or tenderness on palpation.
Examination reveals no other such lesions, or indeed any abnormalities. The adjacent toenails do not appear to be involved.
What is the diagnosis?