Clinical history: 50 year old male lesion on forehead.
What is the diagnosis, the main differential, and pathological features that help to distinguish between them?
Although at first glance this lesion appears to have some features of keratoacanthoma including crateriform architecture, liping, and keratinocytes with eosinophilic cytoplasm,
closer inspection reveals that the lesion is relatively flat, and the 'crateriform architecture' and 'lipping' actually represent papillary projections near the borders of the
lesion combined with intoe-ing of the rete ridges, both features of verruca vulgaris.
In addition, close inspection of the areas showing eosinophilia reveal nuclei which are small
and round with no cytologic evidence of atypia, as well as a smooth basal layer with no atypia.
Other features that support the diagnosis include the presence of hemorrhage above
tips of papilla, alternating hyper and hypo-granulosis, and upon close inspection alternating para and orthokeratosis. The lesion is also papillary although a few of the papillary
projections are not captured in this level, but are suggested by the shape of the vertical hemorrhages in the horn. This is one of the common lesions that can give rise to prominent hyperkeratosis (cutaneous horn).