A Patient With Dermatomyositis and Linear Streaks on the Back

A 27-year-old Chinese man presented with a nonpruritic rash on his thumbs that had developed gradually over a 1.5-year period (Figure 1). Similar lesions subsequently appeared on the antecubital fossae, knees (Figure 2), and popliteal fossae. The lesions were persistent and nonmigratory and were apparently aggravated by heat and sweating. The patient had a younger brother, who was similarly affected. His medical history was otherwise unremarkable. On physical examination, well-demarcated, symmetric, lichenified, erythematous plaques were noted over the dorsa of both hands, the antecubital fossae, the knees, and the popliteal fossae. The palms and soles were unaffected. A punch skin biopsy specimen was obtained from the dorsum of the left hand (Figure 3). What is your diagnosis?