This lesion came from an immunocompromised elderly patient who presented with what clinically appeared to be a squamous cell carcinoma on the arm. At low magnification there is significant endophytic squamous hyperplasia and it certainly looks like a squamous cell carcinoma. There are numerous neutrophilic microabscesses within the epithelium, which are filled with pigmented yeast forms as well as small septate hyphae, consistent with phaeohyphomycosis. I think pigmented or dematiaceous fungi are quite considerate, since they spare the extra expense of performing special stains to identify them.
The differential diagnosis includes chromoblastomycosis, as the yeast forms look like the so called “sclerotic bodies” found in chromoblastomycosis. Chromoblastomycosis will also cause prominent endophytic squamous hyperplasia, mimicking squamous cell carcinoma. Chromoblastomycosis infection lacks hyphae and pseudohyphae, which helps distinguish it from phaeohyphomycosis. If I get follow-up culture results I’ll provide an update.