An 89-year-old man reported that this lesion began developing on his left forearm 11 days earlier. It is a keratoacanthoma, a rapidly growing but benign neoplasm that occurs predominantly on the extensor surfaces of the hands and forearms of white men over age 50. Because it is grossly indistinguishable from squamous cell carcinoma, microscopic diagnosis is mandatory.
Spontaneous regression occurs but may leave a disfiguring scar. Surgical excision (as was done in this case) or electrodessication and curettage is recommended.
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