Answer: D

Tinea corporis (TC), also known as ringworm, is a superficial fungal infection of glabrous skin that usually appears on the arm or legs and is not seen on the palms, soles, or groin.14 Although the disease has existed for millennia, a causative fungus was first described in 1842 by David Gruby.15 He identified Microsporum audouinii as a one of the fungi that causes TC.15

Superficial fungal infections are common, affecting 20% to 25% of the world’s population.16 Factors related to TC infection include younger age, sex, and race, but how these factors correlate with susceptibility is not well known.14 TC is more common in men than in women.14

A compromised immune system increases the risk for TC and other superficial fungal infections.14 Although the prevalence of fungal infections is similar in patients with HIV and non-HIV-infected individuals, the severity of fungal infections is increased in the former group.17 Several genetic polymorphisms in toll-like receptors (TLRs), including TLR1, TLR3, and TLR4, are associated with an increased risk for fungal disease. However, knowledge about the molecular mechanisms that drive an increased predisposition to TC is lacking.18

Various fungal species, or dermatophytes, can cause TC. Certain fungal species are found worldwide, whereas others are more localized to specific geographic regions.14 In the United States, Trichophyton rubrum is the more common cause of TC among adults, and the zoophilic pathogen Microsporum canis infects more children.14,19 These fungi express numerous enzymes and factors that adhere to and spread on the skin.20 They use keratin on the skin as a source of nutrients and a vehicle to spread. In response to the fungi and degraded keratin, the body releases inflammatory mediators, leading to the erythematous, pruritic rash.20 In TC, the ringworm pattern is caused by the fungi rapidly spreading outward, with a reduction or clearance of the fungi in the center of the plaque.14

Risk factors for acquiring TC include humid conditions, excessive sweating, and constrictive clothing that does not allow moisture to escape. The fungi that cause TC normally are found on the skin but can develop into an opportunistic infection.14 TC can spread between people, between pets and humans, and between fomites (ie, bed linens or athletic gear) and skin.13,19 In addition, minor trauma, such as that from close contact or mat and carpet burns, can create an environment that allows fungal species to flourish. Thus, individuals who participate in close-contact sports, such as wrestling, football, and rugby, are more likely to develop TC.21

Clinically, patients with TC present with an annular or serpiginous scaly plaque that has central clearing and an erythematous raised ring on the perimeter. The plaque may be accompanied by pruritus, dry and flaky skin around the rash, or even hair loss within the rash.14 Histologically, septate, branching hyphae in the stratum corneum or skin scrapings are visualized.14 In addition, certain fungal species will fluoresce under Wood lamp examination.14

The differential diagnosis for TC includes erythema annulare centrifugum, nummular eczema, tinea versicolor, cutaneous candidiasis, contact dermatitis, pityriasis rosea, seborrhea, MF, and parapsoriasis.14 Majocchi granuloma, a deeper dermatophyte infection that involves hair follicles and is common in immunocompromised individuals,22 also is included in the differential diagnosis.22 TC is most commonly found on shaved surfaces such as the legs of women who apply topical corticosteroids after skin irritation, thus facilitating a fungal infection.14

TC is diagnosed clinically and can be confirmed with Wood lamp examination, as well as microscopic evaluation of skin shavings displaying branched hyphae of fungal species.14 Skin shaving of the scales can be prepared with a potassium hydroxide preparation, which, along with mild heat, will soften keratin and show hyphae.14 Alternative methods of diagnosis include culturing the skin flora, a dermatophyte test medium that changes color in the presence of dermatophyte, or histologic staining with periodic acid-Schiff or Grocott methenamine silver, which stain fungal elements pink or black, respectively.14

In most cases, treatment with a topical antifungal agent will eliminate the fungal infection. Common agents include fluconazole, griseofulvin, itraconazole, and terbinafine. These usually are applied to the affected area multiple times a day for 2 to 4 weeks.23,24 It is important to continue to treat the region even after the lesion resolves to ensure complete eradication of the fungus.14

For individuals with a compromised immune system or extensive TC, a course of oral antifungal agents can be used and have a good cure rate. Common oral agents include fluconazole, itraconazole, and terbinafine. It is important to note that the oral agents have adverse effects, including liver and renal toxicity, and some are contraindicated in pregnant women.14

The patient in the case was prescribed a topical antifungal, which she applied twice daily for 3 weeks, leading to resolution of the rash on her leg.

Jay Patel, BS, and Yelena Dokic, BSA, are medical students at Baylor College of Medicine, and Christopher Rizk, MD, is a dermatologist affiliated with Elite Dermatology Houston.

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