A 70-year-old man presented with a 4 month history of painful, pruritic rash involving the scalp, face, chest, and back. He had a history of colon cancer and deep venous thrombosis. A biopsy sample of the rash had been interpreted as eczema, but he had had only minimum improvement with short courses of both topical and systemic corticosteroids and antibiotics. Examination showed a healthy-looking man with erythematous papules and plaques displaying cornflake-like scale in a seborrheic distribution.
Oral and ocular mucosae were normal. Previous serological studies, including antinuclear antibody (ANA), complement, and serum protein electrophoresis, were normal. Skin biopsies of this rash reveal keratinocyte acantholysis. What is the most likely diagnosis?
