Erythrasma is a condition of the moist, intertriginous areas of the body which represents a superficial infection of the epidermis with Corynebacterium minutissimum. It is distinguished from tinea cruris by the more diffuse border and by a more diffuse involvement of the central part of the plaque, as opposed to the more healed appearance of tinea cruris.
Erythrasma fluoresces coral pink (or red) on Wood's Lamp inspection due to excess coproporphyrin III, which is a metabolic byproduct of the causative bacteria. Please remember the fluorescence will not be present if the patient recently showered. It is also important to note the bacteria do not fluoresce blue or blue-green, because the organisms that do that, Microsporum canis and M. audouinii, are the causative agents of tinea capitis, but not tinea cruris. For a bonus, what is the most common fungal organism to cause tinea cruris?
The vast majority of you killed this one. Good job.