Superficial dermatophyte (and Malassezia) infections of skin, hair, and nails with site-specific presentations.
Also known as: tinea, ringworm, dermatophytosis, athlete's foot, jock itch, tinea versicolor, pityriasis versicolor
Overview
Superficial fungal infections of keratinized tissue. Dermatophyte tineas (Trichophyton, Microsporum, Epidermophyton) cause classic ringworm of body (corporis), scalp (capitis), feet (pedis), groin (cruris), hands (manuum), and nails (unguium/onychomycosis). Tinea versicolor (pityriasis versicolor) is caused by Malassezia furfur — a yeast, not a true dermatophyte.
Epidemiology
Tinea pedis is the most common dermatophytosis worldwide (~10% point prevalence). Tinea capitis predominantly affects prepubertal children, especially Black children in the US (Trichophyton tonsurans). Tinea versicolor is common in adolescents and young adults in humid climates.
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Question 1DermatologyMedium
A 36-year-old woman has annular plaques with central clearing on the thigh. KOH preparation shows branching septate hyphae. Which of the following is the most appropriate treatment?
ATopical permethrin cream
BTopical terbinafine cream
CTopical mupirocin ointment
DTopical nystatin cream
EOral acyclovir capsules
Reveal answer & full explanation
Correct answer: B — Topical terbinafine cream
ATopical permethrin cream
BTopical terbinafine cream✓
CTopical mupirocin ointment
DTopical nystatin cream
EOral acyclovir capsules
Why Topical terbinafine cream is correct
Annular plaques with central clearing plus septate branching hyphae on KOH confirm tinea corporis.
Terbinafine is fungicidal against dermatophytes and is first-line for localized disease.
A topical agent suffices when only limited, non-hair-bearing skin is involved.
Why the others are wrong
Topical mupirocin ointment — Mupirocin targets gram-positive bacteria for impetigo and has no antifungal action; a rash-treatment trap.
Oral acyclovir capsules — Acyclovir treats herpesvirus, which causes grouped vesicles, not KOH-positive plaques.
Topical permethrin cream — Permethrin kills scabies mites and lice, not dermatophytes.
Topical nystatin cream — nystatin is an antifungal, which makes it tempting, but it acts on Candida and has no useful activity against dermatophytes such as Trichophyton, so it does not clear tinea corporis; a wrong-fungus trap.
Question 2DermatologyMedium
A 36-year-old has annular scaly plaques on the trunk with central clearing. Which of the following diagnostic tests is most appropriate?
ALyme disease serologic testing
BPatch testing for contact allergens
CSerum antinuclear antibody testing
DWood lamp examination of the plaques
EPotassium hydroxide preparation
Reveal answer & full explanation
Correct answer: E — Potassium hydroxide preparation
ALyme disease serologic testing
BPatch testing for contact allergens
CSerum antinuclear antibody testing
DWood lamp examination of the plaques
EPotassium hydroxide preparation✓
Why Potassium hydroxide preparation is correct
Annular scaly plaques with central clearing are classic for tinea corporis, a dermatophyte infection.
KOH preparation of a scale scraping reveals branching septate hyphae and rapidly confirms the diagnosis at the bedside.
It is inexpensive, immediate, and directly tests the suspected organism.
Why the others are wrong
Serum antinuclear antibody testing — ANA screens for systemic autoimmune disease such as lupus; it does not detect a dermatophyte and would not explain annular scale with central clearing.
Patch testing for contact allergens — Patch testing diagnoses allergic contact dermatitis, which lacks the annular central-clearing morphology of tinea.
Wood lamp examination of the plaques — Wood lamp fluorescence is unreliable for most body-surface dermatophytes (Trichophyton does not fluoresce), so KOH remains the test of choice.
Lyme disease serologic testing — erythema migrans can also expand with central clearing, but it lacks scale, and early Lyme disease is diagnosed clinically because antibodies are often not yet detectable in the first weeks; annular scaly plaques point to tinea, which a KOH scraping confirms.
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Family member with tinea capitis (shared combs, hats)
Pathophysiology
Dermatophytes secrete keratinases that allow invasion of stratum corneum, hair, and nail. Inflammation results from host immune response; minimal inflammation in immunosuppressed → 'tinea incognito' under steroids. Malassezia is a lipophilic yeast colonizing sebaceous skin; conversion from yeast to mycelial form produces clinical pityriasis versicolor; azelaic acid produced by the organism inhibits melanocytes (hypopigmentation).
Clinical presentation
Symptoms
Pruritus (mild to moderate)
Scalp scaling, hair loss (capitis)
Burning, fissuring between toes (pedis)
Cosmetic concerns (versicolor)
Signs / physical exam
Tinea corporis: annular erythematous plaque with raised scaly active border and central clearing; satellite lesions if extensive
Tinea capitis: scaly patches with broken hairs ('black dot' pattern: endothrix T. tonsurans has arthroconidia inside the hair shaft, so hairs break at the scalp surface and do not fluoresce; ectothrix Microsporum canis coats the outside of the shaft and fluoresces on Wood's lamp); kerion = boggy inflammatory mass with pustules and lymphadenopathy; favus = yellow scutula crusts
Onychomycosis: yellow/white nail discoloration, subungual hyperkeratosis, onycholysis, dystrophy (distal subungual most common)
Tinea versicolor: hypo- or hyperpigmented finely scaling macules and patches on upper trunk, neck, shoulders; 'fawn-colored' or pink in untanned skin
Classic findings
Active raised scaly border with central clearing (corporis); KOH 'spaghetti and meatballs' (versicolor); Wood's lamp yellow-green fluorescence (Microsporum tinea capitis).
Differential diagnosis
Nummular eczema — Coin-shaped, no central clearing, KOH negative
Psoriasis — Silvery scale, extensor surfaces, nail pitting
Seborrheic dermatitis — Greasy scale in seborrheic distribution
Erythrasma (groin/folds) — Coral-red Wood's lamp fluorescence (Corynebacterium minutissimum); thin brown patches; treat with erythromycin/clindamycin
Granuloma annulare — Smooth annular skin-colored to violaceous papules without scale; KOH negative
Alopecia areata (vs tinea capitis) — Smooth bald patches without scale, exclamation point hairs, no broken hairs at scalp; KOH/culture negative
Vitiligo (vs tinea versicolor) — Complete depigmentation, no scale, Wood's lamp bright milky white fluorescence
Diagnostic workup
Diagnostic criteria
KOH and/or culture positivity in characteristic clinical setting.
Labs
KOH preparation of scale or hair: branching septate hyphae (dermatophyte) or short hyphae + spores ('spaghetti and meatballs') for versicolor
Fungal culture (Sabouraud or DTM agar) — confirms species, especially for capitis and onychomycosis before systemic therapy
Wood's lamp: blue-green fluorescence with Microsporum tinea capitis (T. tonsurans is non-fluorescent); pale yellow with versicolor; coral-red with erythrasma
PAS-stained nail clipping or PCR for onychomycosis diagnosis
Baseline LFTs before oral terbinafine or itraconazole
Imaging
Not indicated
Diagnostic algorithm
Variant
Site / Lesion
Preferred Therapy
Corporis (ringworm)
Annular plaque, raised scaly border, central clearing
Topical azole or terbinafine 2-4 wks
Capitis
Scaly scalp, broken hairs, ± kerion
Oral terbinafine or griseofulvin × 4-8 wks + antifungal shampoo
Pedis
Interdigital maceration; moccasin hyperkeratosis
Topical terbinafine; oral if moccasin/extensive
Cruris
Inguinal scaly plaques sparing scrotum
Topical azole or terbinafine
Onychomycosis
Yellow nail, subungual debris, onycholysis
Oral terbinafine 6 wks (fingers) / 12 wks (toes)
Versicolor
Hypo/hyperpigmented finely scaling macules on trunk
Selenium sulfide or ketoconazole shampoo; oral itraconazole if extensive
Tinea capitis (REQUIRES SYSTEMIC): oral griseofulvin (microsize) 20-25 mg/kg/day × 6-8 weeks (preferred for Microsporum) OR oral terbinafine 3-6 mg/kg/day × 4-6 weeks (preferred for Trichophyton tonsurans, more common in US); adjunctive selenium sulfide or ketoconazole shampoo 2-3x/wk reduces shedding and transmission
Onychomycosis: oral terbinafine 250 mg daily × 6 weeks (fingernails) or 12 weeks (toenails) — most effective; alternatives: itraconazole pulse, fluconazole weekly; topical efinaconazole or tavaborole if oral contraindicated
Tinea versicolor: topical selenium sulfide 2.5% lotion, ketoconazole 2% shampoo, or clotrimazole — lather, leave 10 min, rinse — daily × 1-2 weeks; or oral fluconazole 300 mg weekly × 2 weeks or itraconazole 200 mg/day × 5-7 days for widespread/refractory disease (oral terbinafine is not effective); recurrence is common, so consider monthly prophylactic therapy
Kerion (inflammatory tinea capitis)
Oral antifungal (terbinafine or griseofulvin) started promptly because untreated kerion can scar; a short oral prednisone course may be considered for severe pain or inflammation but has not been shown to prevent scarring
Antibiotic only if bacterial superinfection — DO NOT drain kerion (sterile inflammation)
Tinea incognito (steroid-modified)
Discontinue topical corticosteroid
Treat with appropriate antifungal — often requires systemic therapy because of extensive subclinical spread
Second-line / adjunct
Avoid systemic ketoconazole due to hepatotoxicity and adrenal suppression risk (FDA boxed warning)
Recheck culture and consider drug resistance (e.g., Trichophyton indotineae — emerging terbinafine-resistant species)
Counsel on hygiene: clean and dry skin folds, antifungal foot powder, change socks, treat shoes with antifungal spray
Complications
Scarring alopecia from untreated kerion
Bacterial superinfection (impetigo, cellulitis), especially with maceration
Onychomycosis: chronic relapsing course, gateway for cellulitis in diabetics
Id reaction (autoeczematization): vesicular dermatitis on hands during active tinea pedis — sterile, immune-mediated
Terbinafine adverse effects: hepatotoxicity, taste disturbance, drug interactions (CYP2D6)
Tinea cruris spares the scrotum; candida intertrigo involves the scrotum with satellite pustules — quick distinguishing feature.
Pruritic annular plaque with central clearing and active border = tinea corporis until proven otherwise — KOH first, treat empirically only if KOH unavailable.
NEVER use topical betamethasone/clotrimazole combination (Lotrisone) — the steroid potency is too high for the skin folds where it is commonly used; causes atrophy and tinea incognito.
Hair-follicle dermatophytoses (tinea capitis, tinea barbae, Majocchi granuloma) REQUIRE systemic therapy — topicals will not penetrate hair shaft.
Tinea versicolor responds to therapy but pigment changes can take months to fade — counsel patients to avoid attributing relapse to treatment failure.
References
AAD 1996 — Guidelines of care for superficial mycotic infections of the skin: tinea corporis, tinea cruris, tinea faciei, tinea manuum, and tinea pedis (Drake LA, et al., AAD Guidelines/Outcomes Committee, J Am Acad Dermatol 1996;34(2 Pt 1):282-286)
Cochrane 2016 — Systemic Antifungal Therapy for Tinea Capitis in Children (Chen et al., Cochrane Database Syst Rev 2016)
AAFP 2025 — Diagnosis and Management of Tinea Infections (Caplan AS, Gold JAW, Smith DJ, Ely JW, Am Fam Physician 2025;112(4):382-392)
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