This 28-year old woman presented with a 0.7-centimeter white nodule on her right toe. Acquired digital fibrokeratoma (ADFK) typically occurs on the fingers or toes of adults and presents as an indolent nodule that rarely exceeds 1.5 centimeters in size. It is usually of no clinical significance but may be confused histologically with periungual fibromas seen in tuberous sclerosis. The presence of multinucleated and stellate cells in the latter help distinguish it from ADFK. The differential diagnosis of ADFK also includes an accessory digit, knuckle pad, sclerotic fibroma, and dermatofibroma. The clinical history is key to differentiating an accessory digit and a knuckle pad from ADFK. Moreover, nerve bundles are particularly prominent in an accessory digit. In sclerotic fibroma and dermatofibroma, the collagen bundles are thinner and are usually arranged in a storiform, rather than a haphazard, pattern. Although a traumatic cause is favored by some, the pathogenesis of ADFK is not currently known.
Histologic examination of the lesion reveals a dermal proliferation of dense, hyalinized collagen bundles haphazardly arranged perpendicular to the overlying orthohyperkeratotic epidermis. There are sparse fibroblasts and few blood vessels present.