tinea corporis

Tinea corporis is caused by dermatophytes (Trichophyton, Microsporum, Epidermophyton species) that infect the keratinized stratum corneum of the skin, using keratinase enzymes to invade superficial tissue without penetrating deeper layers. Transmission occurs via direct contact with infected humans, animals (e.g., Microsporum canis from pets), or fomites (contaminated towels, gym mats—hence 'tinea gladiatorum' in wrestlers), and is favored by warm, humid environments. Clinically, it presents as one or more pruritic, well-demarcated, annular (ring-shaped) plaques with raised, scaly, erythematous active borders and central clearing, which can expand centrifugally. Diagnosis is confirmed with a KOH preparation of skin scrapings showing septate, branching hyphae; Wood's lamp examination may show fluorescence with certain Microsporum species. Treatment involves topical antifungals (clotrimazole, terbinafine) for localized disease; extensive, refractory, or immunocompromised cases require oral antifungals (terbinafine, itraconazole, fluconazole). Important board-relevant associations include distinguishing tinea corporis from other annular lesions like nummular eczema, granuloma annulare, and erythema migrans (Lyme disease, which lacks scale), as well as recognizing tinea incognito—an atypical, less scaly presentation caused by inappropriate topical corticosteroid use that masks the classic appearance and can worsen the infection.