Peer Reviewed

Photo Quiz

What Is This Rash on an Older Man’s Groin?

Kathleen E. Kramer, MD1 • Angela M. Crotty, MD2

1Aviation Medicine, Naval Branch Health Clinic Coronado, Naval Medical Center San Diego, Coronado, California
2Department of Dermatology, Naval Medical Center San Diego, San Diego, California

Kramer KE, Crotty AM. What is this rash on an older man’s groin? Consultant. Published online November 10, 2021. doi:10.25270/con.2021.11.00002

Received May 6, 2021. Accepted May 17, 2021.

The authors report no relevant financial relationships.

The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, the Department of Defense, or the US Government.

Kathleen E. Kramer, MD, Naval Branch Health Clinic Coronado, 601 McCain Boulevard, Coronado, CA 92135 (


A 62-year-old man presented to our dermatology clinic with several-year history of a groin rash. It had waxed and waned over the years, with occasional episodes of associated pruritis elicited by periods of increased sweating.

History. He was otherwise healthy with no significant medical history. He reported showering early that morning and had been showering roughly twice daily in an effort to ensure his groin area was clean.

The rash had been treated previously with topical nystatin powder, oral terbinafine, oral itraconazole, topical econazole, and tacrolimus prescribed by his primary care physician. The patient reported transient improvement in the appearance of the rash with application of antifungal creams and over-the-counter moisturizers, but it did not clear completely with those modalities.

Physical examination. The patient had a skin phototype 2, was well-nourished, and was not in acute distress. Upon gross examination, the patient displayed uniformly pink to brown plaques with clearly demarcated borders and minimal overlying scale, extending from the bilateral inguinal creases and involving lateral aspects of the scrotum and proximal inner thighs bilaterally (Figure 1). When examined with a Wood’s lamp, the area showed coral-red fluorescence (Figure 2).

Figure 1. Plaques were noted bilaterally on the patient’s groin.
Figure 1. Plaques were noted bilaterally on the patient’s groin.

Figure 2. The patient’s groin lesions fluoresced when examined via Wood’s lamp.
Figure 2. The patient’s groin lesions fluoresced when examined via Wood’s lamp.



Answer and discussion on next page.

Correct answer: C. Erythrasma

Erythrasma is a bacterial skin infection caused by excessive proliferation of Corynebacterium minutissimum in the stratum corneum.1 The most common sites are the fourth interdigital toe space, inframammary folds, and axillae, but it can also be found in the groin. The organism that causes erythrasma produces porphyrins, which fluoresce under Wood’s lamp examination.2 This can help distinguish erythrasma from other etiologies, although fluorescence is not always seen. False negatives are particularly common upon Wood’s lamp examination if the area has been recently cleansed.3

Upon initial examination, erythrasma of the groin may be mistaken for other clinical entities such as tinea cruris, seborrheic dermatitis, inverse psoriasis, or intertrigo.3 It can be readily distinguished if the classic examination finding of coral-red fluorescence on Wood’s lamp examination is present. Erythrasma is important to include on the diagnostic differential diagnosis for groin rash, particularly when it does not improve with various courses of oral and topical antifungal treatments.

Treatment and management. Erythrasma is a superficial skin infection caused by C minutissimum, which is readily treatable with topical antibiotic therapy. It can demonstrate significant clinical overlap with other entities similar in appearance, including dermatophyte infection, candidiasis, inverse psoriasis, seborrheic dermatitis, and eczematous dermatitis, among other possible causes. The appearance of erythrasma can vary, but it is typically more uniformly pink to brown and lacks prominent scale. Dermatophyte infections classically have a scaly red border with central clearing, although this is not always present in moist sites such as the groin.4 Dermatophyte infections will typically improve with antifungal treatments. As our patient’s condition did not improve with antifungal treatments, it provided an important clue to an alternative diagnosis. Inverse psoriasis plaques especially may be entirely indistinguishable from erythrasma.5

The clinical overlap may lead to delays in diagnosis and effective treatment. Wood’s lamp examination is a low-risk evaluation tool with high potential clinical benefit in initial evaluation of intertriginous lesions, as well as those that have not responded to other therapies. While a simple and effective tool, Wood’s lamp is subject to false negatives in cases where patients have recently cleansed, as porphyrins are water soluble. It is also subject to false positives in the case of clothing fibers or soap residues.5

Other clinical clues can also point to a diagnosis of erythrasma in the absence of Wood’s lamp examination. Predisposing factors for erythrasma include diabetes6, obesity7, humid environment, heavy sweating, poor hygiene, and advanced age.8 The appearance of erythrasma can vary but differs slightly from tinea infections and is typically more uniformly pink to brown, whereas tinea infections tend to have a scaly red border with central clearing.4

Topical medications such as clindamycin or erythromycin are typically effective for treating erythrasma.2 Oral treatments such as a single dose of clarithromycin, 1 g or a 2-week course of erythromycin or tetracycline are also effective and well tolerated.9 In settings where a Wood’s lamp is not available, it may be prudent to trial an empiric course of topical clindamycin prior to more expensive specialty visits.


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