A 17-year-old girl presents with a rash and itching in the groin (Figure 124-1). On examination, she was found to have an expanding scaly erythematous well-demarcated ring in the inguinal area. A skin scraping was treated with Swartz-Lamkins stain and the dermatophyte was highly visible under the microscope (Figure 124-2). The clinician diagnosed tinea cruris. She was offered a choice between an oral or topical antifungal medicine and she preferred to have the systemic treatment. She was given 2 weeks of oral terbinafine 250 mg daily and her tinea cruris resolved.

Tinea cruris is an intensely pruritic superficial fungal infection of the groin and adjacent skin.

Crotch rot and jock itch.

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  Using data from the National Ambulatory Medical Care Survey and the National Hospital Ambulatory Medical Care Survey (NHAMCS) (1995–2004), there were more than 4 million annual visits for dermatophytoses and 8.4 percent were for tinea cruris.¹

  
  
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  Tinea cruris is more common in men than women (three-fold) and rare in children. However, it may be seen in teens after puberty.

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  Most commonly caused by the dermatophytes: Trichophyton rubrum, Epidermophyton floccosum, Trichophyton mentagrophytes, and Trichophyton verrucosum. T. rubrum is the most common organism.²

  
  
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  Can be spread by fomites, such as contaminated towels.

  
  
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  The fungal agents cause keratinases, which allow invasion of the cornified cell layer of the epidermis.²

  
  
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  Autoinoculation can occur from fungus on the feet or hands.

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  Wearing tight-fitting or wet clothing or underwear has traditionally been suggested; however, in a study of Italian soldiers, none of the risk factors analyzed (e.g., hyperhidrosis, swimming pool attendance) were significantly associated with any fungal infection.³

  
  
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  Obesity and diabetes mellitus may be risk factors.⁴

Clinical Features

The cardinal features are scale and signs of inflammation. In light-skinned persons, inflammation often appears pink or red and in dark-skinned persons, the inflammation often leads to hyperpigmentation (see Figure 124-4). Occasionally, tinea cruris may show central sparing with an annular pattern as in Figure 124-1, but most often is homogeneously distributed as in Figures 124-3 and 124-4. If the tinea cruris was missed and the patient was given topical steroids to treat the itching, the tinea cruris can become tinea incognito and the eruption can expand down the thighs (Figure 124-5). Tinea incognito often shows concentric rings within the scaling plaques.