[Ten clues to the histopathologic diagnosis of infectious skin diseases].

Specific diagnoses of infectious skin diseases can be established in dermatopathology by several methods: by algorithms based on pattern analysis, by differential diagnosis of clinicopathologically similar disorders, by attention to pitfalls, by exceptions to the rules and finally, very elegantly by clues. Ten clues to specific diagnoses of infectious skin diseases which are of importance in routine practice are presented. Clue 1: Features of an angiocentric angiodestructive lymphoma in the centre of a wedge-shaped necrotic insect bite reaction are a clue to a bite by a spider. Clue 2: Parakeratosis with spiky imprints on the surface above features of an insect bite reaction are a clue to scabies. Clue 3: A faintly eosinophilic section of a skin specimen with prominent fibrin thrombi and extravasated erythrocytes are a clue to a septic fungal vasculitis. Clue 4: Infectious agents in association with otherwise typical features of leukocytoclastic vasculitis are a clue to septic vasculitis. Clue 5: Steel-grey nuclei with marginated nucleoplasm of keratinocytes are a clue to the diagnosis of early infection by herpes virus. Clue 6: Owleye-like cells are a clue to infection by cytomegalovirus. Clue 7: Detection of Entamoeba histolytica in perianal and perineal ulcers are a clue to amebiasis. Clue 8: Round-oval to rectangular spaces within the stratum spinosum of the epidermis are a clue to larva migrans. Clue 9: Macrophages and plasma cells in association with otherwise typical features of pityriasis lichenoides are a clue to secondary syphilis. Clue 10: A lichenoid infiltrate of lymphocytes and plasma cells are a clue to acrodermatitis chronica atrophicans.