Wood's lamp findings in skin infection (erythrasma)

What it is

Erythrasma is caused by a Corynebacterium minutissimum infection. A Wood's lamp may assist with the diagnosis of certain skin conditions, and it is the bedside procedure to name when the question asks what will aid the diagnosis of a scaling or a depigmented rash.

How it presents

The presentation described is a 69-year-old female with scaling, redness and irritation under her breasts for the past several months, who has tried several over-the-counter antifungal creams without any improvement. On examination there are erythematous, well demarcated patches with some scale under both breasts.

Two features of that history are what point away from a fungal infection and toward erythrasma: the site is a skin fold, and the antifungal creams did nothing.