What is your diagnosis?

An 88-year-old woman with Alzheimer disease and a history of numerous basal cell carcinomas presented to our clinic with a rapidly growing nodule on the right arm of 2 weeks' duration. No antecedent lesion was present at that site. The nodule was asymptomatic, and a review of systems did not reveal constitutional symptoms. Physical examination revealed a 3×2-cm,oval, dome-shaped, burgundy-colored nodule with a smooth shiny surface adjacent to the right antecubital fossa. A round, 0.5-cm, crateriform ulceration was present at the right pole of the nodule.