Dermatology · Fungal Infections
The facts most likely to be tested
Tinea corporis presents as an annular, erythematous, scaly plaque with a central clearing and an active, raised border.
The gold standard for diagnosis is a potassium hydroxide (KOH) preparation showing septate hyphae.
The most common causative pathogen is Trichophyton rubrum.
First-line treatment for localized disease is topical antifungal therapy, specifically terbinafine or clotrimazole.
Topical corticosteroids are contraindicated as they can cause tinea incognito, which masks the classic appearance and promotes fungal growth.
Oral antifungal therapy, such as oral terbinafine or itraconazole, is reserved for extensive disease, immunocompromised patients, or cases refractory to topical treatment.
The infection is transmitted via direct contact with infected skin, animals, or fomites.
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A 24-year-old male wrestler presents to the clinic with a persistent rash on his trunk. Physical examination reveals a 3-cm annular, erythematous plaque with a scaly, raised border and central clearing. He reports mild pruritus at the site. He has been applying a topical hydrocortisone cream for the past week with minimal improvement. The patient denies any systemic symptoms or recent travel.
What is the most appropriate next step in management?
Discontinue topical corticosteroids and initiate topical terbinafine.
The patient presents with classic tinea corporis; the use of topical steroids is contraindicated as it leads to tinea incognito, and the correct management is to switch to a topical antifungal.
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Etiology / Epidemiology
Caused by dermatophytes (e.g., Trichophyton rubrum). High risk in athletes and those with skin-to-skin contact.
Clinical Manifestations
Presents as an annular lesion with central clearing and an active, scaly border.
Diagnosis
KOH preparation is the gold standard, revealing septate hyphae.
Treatment
Topical azoles (e.g., clotrimazole) are first-line. Avoid topical steroids as they cause tinea incognito.
Prognosis
Excellent prognosis with topical therapy; recurrence is common if fomites are not addressed.
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Epidemiology & Etiology
Commonly caused by Trichophyton rubrum, Microsporum canis, or Epidermophyton floccosum. Transmission occurs via direct contact with infected humans, animals, or contaminated fomites. High prevalence in wrestlers and individuals in humid, tropical climates.
Pertinent Anatomy
Infection is restricted to the stratum corneum of the epidermis. The fungus does not penetrate deeper into living tissue due to the presence of serum inhibitory factors.
Pathophysiology
Dermatophytes produce keratinases that digest keratin, allowing the fungus to colonize the superficial skin layers. The host immune response triggers an inflammatory reaction at the periphery, leading to the characteristic annular morphology. The center clears as the fungus moves outward to find fresh keratin.
Clinical Manifestations
Classic presentation is an annular, erythematous patch with a raised, scaly border and central clearing. Patients often report pruritus. Tinea incognito occurs when topical steroids are applied, masking the classic border and leading to a more diffuse, atypical appearance.
Diagnosis
The KOH preparation is the gold standard diagnostic test. Microscopic examination reveals septate hyphae and arthroconidia. Fungal culture is rarely required unless the diagnosis is uncertain or the infection is recalcitrant.
Treatment
Topical azoles (e.g., clotrimazole, ketoconazole) are the first-line treatment. Apply for 1-2 weeks, continuing for 1 week after resolution. Do not use topical corticosteroids as they exacerbate the infection. Oral antifungals like terbinafine are reserved for extensive or refractory cases.
Prognosis
Prognosis is excellent with adherence to topical therapy. Secondary bacterial infection (e.g., cellulitis) is a rare complication. Recurrence is common if the source of infection is not eliminated.
Differential Diagnosis
Nummular eczema: lacks central clearing and scaly border
Pityriasis rosea: starts with a herald patch and follows a Christmas tree distribution
Granuloma annulare: smooth, non-scaly border
Psoriasis: thick, silvery scales on extensor surfaces
Lyme disease: erythema migrans lacks scale and has a history of tick exposure