Answer: Syphillis
Syphillis IHC

Histology: The skin shows neutrophils within the superficial keratin layer. There is hyperplastic squamous epithelium with pseudoepitheliomatous hyperplasia. In the dermis is a prominent plasma cell infiltrate. An immunohistochemical stain against syphilis shows innumerable red filamentous organism consistent with Treponema Pallidum.

Discussion: The finding of psoriasiform hyperplasia with a dense plasma cell infiltrate in the genital regions should always prompt a differential diagnosis of syphilis. In the past, a Warthin-Starry stain was the preferred method to identify organisms histologically. However, it was often difficult to read this stain and distinguish organisms from non-specific background staining. Currently, the best method of finding the organisms is with immunohistochemistry which is very specific and also easier to read. In the current case, it is teeming with organisms consistent with secondary syphilis. In addition to plasma cells, it is common to have associated acute inflammation in the epidermis which, if not recognized as a component of syphilis, could be misconstrued to be suggestive of a fungal as opposed to a syphilitic infection.