Step by Step
Org
The organisms — three genera
Dermatophyte infections are caused by three related fungal genera — Trichophyton, Microsporum, and Epidermophyton — that all specialize in digesting keratin, the protein found in skin, hair, and nails.
Dx
Diagnosis — KOH prep
A KOH (potassium hydroxide) prep dissolves skin cells but leaves fungal structures intact, making branching hyphae visible under the microscope on a skin scraping — the standard way to confirm a dermatophyte infection.
Name
Naming by location, not organism
The word "tinea" is always paired with a Latin term for body location, not the specific fungal species — tinea pedis (foot), tinea capitis (scalp), tinea cruris (groin), tinea corporis (body), and tinea unguium (nails) can all be caused by the same organisms; only the site differs.
Rx
Treatment depends entirely on location
Tinea pedis, corporis, and cruris usually respond to topical antifungals like clotrimazole or miconazole. But tinea capitis (scalp) and tinea unguium (nails) require oral treatment — griseofulvin or terbinafine — because topical medication can't penetrate deep enough into the hair follicle or nail bed to clear the infection.
Applied Walkthrough
1
A patient has itchy, scaling skin between their toes. This is tinea pedis (athlete's foot) — the most common presentation, typically interdigital — and it usually clears with a topical antifungal like clotrimazole.
2
A child develops patchy hair loss with scaling on the scalp. This is tinea capitis. Unlike tinea pedis, a topical cream alone won't work here — the infection lives inside the hair follicle, so oral griseofulvin or terbinafine is required.
3
An adult has a thickened, yellowed toenail. This is tinea unguium (onychomycosis) — again requiring oral terbinafine, since topical treatment can't penetrate the nail bed effectively.
4
All three cases could be caused by the same organism, like Trichophyton — the location, not the organism, is what determines both the name and the required treatment route.
Exam Application
Exams test whether you know that tinea infections are named by body location rather than by organism, and specifically whether you know which sites (capitis, unguium) require oral treatment versus which (pedis, corporis, cruris) can be treated topically.
⚠ Common Trap
The most common trap is assuming any tinea infection can be treated with a topical cream. Tinea capitis and tinea unguium specifically require oral antifungals, because the infection sits inside the hair follicle or nail bed where topical medication can't reach effectively.
✓ Quick Self-Check
1. What are the three genera that cause dermatophyte (tinea) infections?
Trichophyton, Microsporum, and Epidermophyton.
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2. How is a dermatophyte infection diagnosed at the bedside?
A KOH prep of a skin scraping, which dissolves skin cells and reveals branching hyphae under the microscope.
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3. Is tinea named after the organism or the body location?
The body location — the same organism can cause tinea pedis, capitis, cruris, corporis, or unguium depending on where it infects.
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4. Which two tinea presentations require oral rather than topical treatment, and why?
Tinea capitis and tinea unguium — because the infection is inside the hair follicle or nail bed, which topical medication can't penetrate.
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5. What oral medications are used for tinea capitis and tinea unguium?
Griseofulvin or terbinafine.
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