Puzzling plaque in unexposed spot

It might be psoriasis but the usual areas of scalp, elbows and knees are clear. Fungal infection could come to mind but there is no central clearing and any skin scrapings would be negative.

This is a localised T-cell lymphoma of the skin, sometimes known as mycosis fungoides.

The only way to confidently diagnose this is by doing a skin punch biopsy. This will show atypical lymphocytes invading the epidermis, hence the scale, and forming small Pautrier microabscesses.

Thin plaques respond to UV light