The rash of dermatomyositis is a violaceous erythema with specific signs (Gottron, heliotrope), not the thin, slightly scaly, orange-red plaques of LPP.
LPP is considered a precursor or early stage of MF; it is distinguished from frank MF by the absence of a clonal T-cell population on biopsy and lack of progression to thicker tumors.
A psoriatic plaque is much thicker, more indurated, and has a characteristic silvery scale, whereas an LPP plaque is very thin with fine, "cigarette paper" wrinkling.
Tinea corporis is a fungal infection with an active, advancing border and central clearing, which is confirmed by a KOH test.
