The family physician (FP) agreed that this could be a tinea cruris infection. He performed a potassium hydroxide (KOH) preparation, but did not see any hyphae or fungal elements. (See video on how to perform a KOH preparation here.)
He told the patient that there was no evidence of fungus under the microscope, and took out his Woods lamp (ultraviolet light) to check for erythrasma. The involved area fluoresced a coral red, confirming the diagnosis of erythrasma. Erythrasma is a bacterial infection caused by Corynebacterium minutissimum.
Treatment options include topical erythromycin, topical clindamycin, oral erythromycin, or oral clarithromycin. The patient decided to take oral erythromycin and the FP prescribed 250 mg twice a day for 2 weeks. At a follow-up visit one month later, the rash had completely resolved.
Photos and text for Photo Rounds Friday courtesy of Richard P. Usatine, MD. This case was adapted from: Usatine R, Smith M. Tinea cruris. In: Usatine R, Smith M, Mayeaux EJ, et al, eds. Color Atlas of Family Medicine. 2nd ed. New York, NY: McGraw-Hill; 2013:795-798.
To learn more about the Color Atlas of Family Medicine, see: www.amazon.com/Color-Family-Medicine-Richard-Usatine/dp/0071769641/
You can now get the second edition of the Color Atlas of Family Medicine as an app by clicking on this link: usatinemedia.com