Keratoacanthoma. Light micrograph of a section through a keratoacanthoma nodule. The dark spots are the nucleus of cells, with the double-lobed ones being white blood cells responding to inflammation. The cause of a keratoacanthoma lesion is unknown but it has many features of a viral condition, and consists of a localised proliferation of squamous cells that forms a cratered nodule. The nodule grows over several weeks before gradually disappearing. However, the unsightly nodule is often surgically removed. The cause of keratoacanthoma is unknown, although exposure to sunlight appears to be a factor.