Clinical history: A 49 year old man presented with a solitary scalp lesion. His past medical history is unremarkable. An excisional biopsy was performed.
What is your diagnosis?
Discussion and answer
The nodule is composed of nests of basaloid cells in close apposition, arrayed in a complex pattern that has been likened to a jigsaw puzzle. A rim of densely eosinophilic, PAS-positive basement membrane material envelopes the individual nests and droplets of similar composition are scattered in the centres of the nests.