Tinea pedis, tinea cruris and tinea unguium — Viva

Q1: Definition and classification (2 min)

Define tinea pedis, tinea cruris and tinea unguium. How are they classified?

Model answer. Tinea pedis is dermatophyte infection of the feet, classified as interdigital (commonest), moccasin-type (chronic hyperkeratotic), vesicobullous/inflammatory, and acute ulcerative. Tinea cruris is dermatophyte infection of the groin and upper medial thighs, typically sparing the scrotum and penis. Tinea unguium or onychomycosis is dermatophyte infection of the nail apparatus, classified as distal-lateral subungual (DLSO), superficial white (SWO), proximal subungual (PSO) and total dystrophic (TDO) patterns. All three share the warm, occluded foot-and-shoe reservoir.

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Q2: Pathophysiology and microbiology (3 min)

What organisms are responsible, and why is the foot a reservoir for groin and nail disease?

Model answer. The dominant organism is Trichophyton rubrum. Trichophyton mentagrophytes, T. interdigitale and Epidermophyton floccosum are also important. The emerging Trichophyton indotineae is a terbinafine-resistant species of the T. mentagrophytes complex. Dermatophytes secrete keratinases and proteases that digest keratin. The foot inside a shoe is warm, humid and occlusive, allowing the organism to persist and spread. The patient then autoinoculates the groin when dressing and the nail through direct extension or trauma. The nail plate is a protected reservoir because topical agents and immune cells penetrate poorly.

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Q3: Clinical presentation (3 min)

Describe the four patterns of tinea pedis and the "two feet, one hand" sign.

Model answer. Interdigital tinea pedis produces maceration, scaling and fissuring of the toe webs, especially the 4th web space, and is the portal for bacterial cellulitis. Moccasin-type produces diffuse, fine, silvery scaling on the plantar surface, heels and sides of the feet. Vesicobullous type is an acute, intensely pruritic eruption of vesicles and bullae on the arch or instep. Acute ulcerative type is a painful, malodorous, macerated ulceration in the toe webs, often with bacterial superinfection. The "two feet, one hand" sign is bilateral tinea pedis with unilateral tinea manuum caused by scratching the feet with one hand; the foot is the reservoir and must be treated to clear the hand.

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Q4: Investigations (2 min)

How do you confirm dermatophyte infection? What is the most sensitive test for onychomycosis?

Model answer. KOH wet mount of skin scrapings or subungual debris shows branching septate hyphae. Fungal culture identifies the species and allows susceptibility testing, but takes 2–6 weeks. Periodic acid-Schiff (PAS) stain of nail clippings is the most sensitive single test for onychomycosis. For suspected T. indotineae, molecular identification by ITS sequencing is required because the organism is morphologically indistinguishable from the T. mentagrophytes complex.

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Q5: Differential diagnosis (3 min)

What are the main differentials of tinea pedis, tinea cruris and onychomycosis?

Model answer. For tinea pedis: erythrasma (coral-red Wood's lamp fluorescence), contact dermatitis, atopic dermatitis, dyshidrotic eczema, pitted keratolysis and juvenile plantar dermatosis. For tinea cruris: candidal intertrigo (involves scrotum, satellite pustules), erythrasma, contact dermatitis and psoriasis. For onychomycosis: psoriasis, trauma, lichen planus, yellow nail syndrome, chronic paronychia, and malignancy — especially subungual melanoma or squamous cell carcinoma. Pigmented or single-nail dystrophy should be biopsied before labelling it as fungal.

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Q6: Management (3 min)

Outline the stepwise management of tinea pedis, tinea cruris and onychomycosis.

Model answer. For skin disease, use topical allylamines such as terbinafine 1% (preferred, shorter course) or topical azoles for 1–4 weeks, continuing 1 week after clearance. For extensive moccasin-type or refractory disease, oral terbinafine 250 mg daily for 2–4 weeks or itraconazole pulse is used. For onychomycosis, first-line is oral terbinafine 250 mg daily for 6 weeks (fingernails) or 12 weeks (toenails), with baseline liver function tests. Alternatives include itraconazole continuous or pulse therapy, or topical efinaconazole/tavaborole for 48 weeks in mild distal disease or when systemic therapy is contraindicated. Always treat the foot reservoir, debride thickened nails, and give hygiene advice.

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Q7: Special situations and complications (2 min)

What complications should you look for, and how does management change in diabetes or suspected T. indotineae?

Model answer. Complications include secondary bacterial infection, cellulitis, lymphangitis, dermatophytid reaction, tinea incognito from steroid use, and recurrence. In diabetes, interdigital tinea pedis and onychomycosis are portals for recurrent cellulitis and foot ulceration; treat actively and involve the diabetic foot team. Suspect T. indotineae in extensive, inflammatory, treatment-resistant disease, especially with recent travel to South Asia. Terbinafine is often ineffective due to SQLE mutations; itraconazole, voriconazole or combination therapy may be used with expert mycology support and molecular identification.

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Q8: Prognosis and prevention (2 min)

What is the prognosis for onychomycosis, and what preventive advice do you give?

Model answer. Skin disease usually responds well. Onychomycosis responds to oral terbinafine, but clinical cure takes many months because toenails grow slowly. Recurrence is common. Prevention includes keeping feet dry, drying between the toes, changing socks daily, wearing breathable footwear, rotating shoes, avoiding barefoot walking in communal areas, not sharing towels or nail clippers, hot-washing socks and towels, and treating infected household contacts simultaneously.^[2][3]

References7ShowHide

1. [1]Shi TW, Zhang JA, Zhang XW, et al. Combination treatment of oral terbinafine with topical terbinafine and 10% urea ointment in hyperkeratotic type tinea pedis Mycoses, 2014.PMID 24697872
2. [2]Tausch I, Bräutigam M, Weidinger G, et al. Evaluation of 6 weeks treatment of terbinafine in tinea unguium in a double-blind trial comparing 6 and 12 weeks therapy Br J Dermatol, 1997.PMID 9205509
3. [3]Leung AK, Barankin B, Lam JM, et al. Tinea pedis: an updated review Drugs Context, 2023.PMID 37415917
4. [4]Gupta AK, Susmita, Nguyen HC, et al. Trichophyton indotineae: Epidemiology, antifungal resistance and antifungal stewardship strategies J Eur Acad Dermatol Venereol, 2026.PMID 40613321
5. [5]Leung AK, Barankin B, Lam JM, et al. Tinea pedis: an updated review Drugs Context, 2023.PMID 37415917
6. [6]Sonego B, Corio A, Mazzoletti V, et al. Trichophyton indotineae, an Emerging Drug-Resistant Dermatophyte: A Review of the Treatment Options J Clin Med, 2024.PMID 38930086
7. [7]Gupta AK, Polla Ravi S, Talukder M, et al. Effectiveness and safety of oral terbinafine for dermatophyte distal subungual onychomycosis Expert Opin Pharmacother, 2024.PMID 38221907