This is a nice example of a clear cell acanthoma from the upper thigh of a middle-aged woman. Most of these uncommon lesions we see come from some part of the lower extremity, although certainly not restricted to this site. There are some psoriasiform changes of the epidermis, with few neutrophils in the parakeratotic scale, thinning of the suprapapillary plates, and epidermal hyperplasia. Some reports demonstrate dermatoscopic similarities of clear cell acanthoma and psoriasis. I can see how this flat lesion could look like a psoriatic plaque clinically, while some lesions are more papillated and acanthotic. You can usually see prominent clear cell change on H&E, however, the PAS-stain also really highlights the prominent glycogen in the cytoplasm, highlighting the epidermis that lacks phosphorlyase and thus cannot break down glycogen. Dr. Mihm and his colleague Dr. Caplan have posted a nice review of a case showing a lesion that is more papillated, showing the variability of these lesions.
J Eur Acad Dermatol Venereol. 2003 Jul;17(4):452-5. Psoriasis-like dermoscopic pattern of clear cell acanthoma. Bugatti L, Filosa G, Broganelli P, Tomasini C.
Dermatology. 2001;203(1):50-2. The dermatoscopic pattern of clear-cell acanthoma resembles psoriasis vulgaris. Blum A, Metzler G, Bauer J, Rassner G, Garbe C.