The one incorrect statement is choice “a,” since the history and appearance of the lesion are quite consistent with keratoacanthoma—considered by most to be a low-grade form of squamous cell carcinoma (SCC). All the other statements are true.
Keratoacanthomas (KAs) are quite common, especially in older patients with fair, sun-damaged skin. They appear on areas of the skin that have been directly exposed to the sun. The dorsal forearm is especially typical, but KAs can also appear on the face, hands, shoulders, and back.
The relatively rapid growth is in marked contrast to most other skin cancers and has been linked to the human papillomavirus DNA, which has been found in these lesions. However, by no means is this connection universally accepted as the cause, even though immune suppression, in the susceptible patient, does appear to play a role.
Microscopically, KAs bear a marked resemblance to SCCs. Indeed, they have been known to metastasize in rare instances, although most will involute (albeit with minor scarring) on their own, without treatment. The standard of treatment in this country is to remove KAs surgically and to submit the tissue for pathologic examination, which would officially show “SCC, KA type,” or “well-differentiated SCC.”
The differential diagnosis includes wart, invasive SCC, Merkel cell carcinoma, and melanoma. KAs have nothing to do with bacterial infection.