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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Dermatophytes infect keratinized tissue: skin, hair, nails; KOH shows branching septate hyphae
- Tinea corporis/cruris/pedis: annular scaly plaques or interdigital scaling; topical terbinafine or azole for most localized disease
- Tinea capitis and onychomycosis require oral therapy because topical agents do not penetrate hair shaft/nail adequately
- Tinea versicolor is Malassezia: hypo/hyperpigmented fine-scaling trunk patches; KOH shows spaghetti and meatballs
- Candida favors moist intertriginous areas with beefy red plaques and satellite pustules; pseudohyphae on KOH
Overview
Superficial fungal skin infections are common and highly testable because morphology and KOH findings distinguish organisms. Dermatophytes such as Trichophyton, Microsporum, and Epidermophyton cause tinea corporis, pedis, cruris, manuum, capitis, and onychomycosis. Tinea versicolor is caused by Malassezia, a lipophilic yeast, and affects pigmentation rather than causing deep inflammation. Cutaneous candidiasis occurs in moist occluded sites and is common with diabetes, obesity, antibiotic use, pregnancy, and immunosuppression.
Epidemiology
Tinea pedis is common in adolescents and adults, especially with occlusive footwear, communal showers, and hyperhidrosis. Tinea capitis is mainly a pediatric disease. Tinea cruris is common in males and often coexists with tinea pedis. Onychomycosis prevalence rises with age, diabetes, trauma, and peripheral vascular disease. Tinea versicolor is more common in warm humid environments. Candidal intertrigo is common in skin folds, diaper area, and under breasts.
Clinical Features
Symptoms
Tinea corporis/cruris: pruritic annular plaque with scaly advancing border and central clearing
Tinea pedis: itching, scaling, fissuring between toes or moccasin distribution on soles
Tinea capitis: scalp scaling, alopecia, broken hairs, lymphadenopathy; boggy kerion may occur
Tinea versicolor: usually mild itch with pigment change on trunk/shoulders
Candida: soreness/burning in moist folds with satellite pustules
Signs
Dermatophyte annular lesions with active scaly edge
Tinea incognito: atypical extensive dermatophyte infection worsened by topical steroids
Onychomycosis: thickened yellow brittle nails with subungual debris
Tinea versicolor: hypo- or hyperpigmented finely scaling macules/patches on chest/back; scale accentuated by scraping
Candidal intertrigo: beefy red moist plaques with satellite pustules in folds
Investigations
First-line
KOH preparationDermatophytes: branching septate hyphae. Tinea versicolor: short hyphae and spores (spaghetti and meatballs). Candida: budding yeast and pseudohyphae
Clinical distributionBody site helps diagnosis: capitis hair, pedis feet, cruris groin sparing scrotum, versicolor trunk, candida folds
Second-line
Fungal cultureUseful for tinea capitis, treatment failure, suspected resistant dermatophyte, or uncertain diagnosis
Wood lampSome Microsporum tinea capitis fluoresces blue-green; tinea versicolor may fluoresce yellow-green/copper-orange but sensitivity is limited
Nail testingConfirm onychomycosis with KOH, PAS stain, culture, or PCR before oral antifungals because mimics are common
Specialist
Diabetes/HIV/immunosuppression evaluationConsider when candidiasis is recurrent/severe, dermatophyte infection is extensive/refractory, or opportunistic infections occur
1
Dermatophytosis
- Localized tinea corporis/cruris/pedis: topical terbinafine, butenafine, clotrimazole, ketoconazole, or other azole; continue beyond clinical clearing as directed
- Tinea capitis: oral griseofulvin or terbinafine; add selenium sulfide or ketoconazole shampoo to reduce transmission
- Kerion: oral antifungal; consider short systemic steroid in severe inflammatory cases under guidance; do not incise as abscess
- Onychomycosis: oral terbinafine is first-line for many dermatophyte infections after confirmation; check LFTs and interactions
2
Tinea versicolor
- Topical selenium sulfide, ketoconazole shampoo, zinc pyrithione, or topical azole for localized disease
- Oral fluconazole or itraconazole for extensive/refractory disease; avoid oral ketoconazole due to hepatotoxicity risk
- Pigment changes can persist for weeks to months after organism eradication
3
Cutaneous candidiasis
- Keep folds dry, reduce friction/occlusion, address diabetes/obesity/antibiotic exposure when possible
- Topical azole or nystatin for intertrigo; barrier paste for diaper dermatitis
- Oral fluconazole for extensive, recurrent, or refractory candidiasis when appropriate
4
Avoiding pitfalls
- Avoid topical corticosteroid monotherapy on undiagnosed annular scaly plaques; it can cause tinea incognito
- Treat tinea pedis to reduce recurrent cellulitis portal of entry
- Assess household contacts and fomites in tinea capitis; avoid sharing combs/hats
Complications
- Tinea incognito: Steroid-modified dermatophyte infection with atypical spread
- Kerion: Inflammatory tinea capitis that can scar and cause permanent alopecia
- Recurrent cellulitis: Toe-web tinea fissures create bacterial portal
- Nail dystrophy: Onychomycosis can impair gait and predispose diabetic foot problems
- Persistent dyspigmentation: Tinea versicolor pigment changes lag behind microbiologic cure
- Recurrent candidiasis: May indicate diabetes, immunosuppression, or ongoing moisture/occlusion
USMLE Step 2 CK Exam Tips
- 1Annular scaly plaque with central clearing = tinea corporis; do KOH before steroids
- 2KOH dermatophyte = branching septate hyphae
- 3Tinea versicolor KOH = spaghetti and meatballs; pigment may remain after treatment
- 4Candida in folds = beefy red plaques with satellite pustules and pseudohyphae
- 5Tinea capitis requires oral therapy; topical shampoo alone is not enough
- 6Kerion is inflammatory fungal infection, not bacterial abscess; do not incise and drain
- 7Tinea cruris typically spares the scrotum; candidiasis often involves scrotum/folds with satellites
- 8Oral terbinafine for onychomycosis: confirm diagnosis first and consider liver monitoring
practicetest your knowledge on fungal skin infectionsApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — dermatology and beyond.
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