Derm Dx: A well-demarcated erythematous patch

A 76-year-old woman who resides in an extended-care facility presents with a several-month history of rash involving her right buttock. The rash has not responded to therapy with topical ketoconazole and triamcinolone creams. The patient is moderately obese and has diabetes. Her medical history is negative for skin cancer, psoriasis, and eczema. Physical examination reveals a well-demarcated, slightly indurated erythematous ...