Before We Start
Location decides the name AND the treatment route
Unlike most infections named after their causative organism, tinea infections are named after WHERE they occur — and that same location detail turns out to be exactly what determines whether a topical cream will be enough, or whether oral treatment is required instead.
💡 Memory Trick
Tinea = dermatophyte ringworm. Named by body location: capitis (scalp) · pedis (foot) · cruris (groin) · unguium (nails). The same organism, different site, sometimes a completely different treatment route.
The Key Points
The organisms, diagnosis, the naming convention, and why treatment splits by site
Org
The organisms — three related 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.
🦠 The shared specialization in digesting keratin explains why these three genera all cause skin, hair, and nail infections specifically, rather than infecting deeper tissue.
Dx
Diagnosis — the 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 at the bedside.
🦠 A KOH prep showing clear branching hyphae, with the surrounding skin cells dissolved away, provides fast bedside confirmation without needing to wait for a fungal culture to grow.
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 organism; only the site differs.
🦠 The exact same Trichophyton species could cause tinea pedis on one patient and tinea capitis on another — the organism identity doesn't change, only the infection site and, correspondingly, the name applied to it.
Rx
Treatment depends entirely on location, not on the organism
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 simply can't penetrate deep enough into the hair follicle or nail bed to clear the infection.
🦠 The exact same organism causing tinea pedis (treatable topically) versus tinea capitis (requiring oral therapy) demonstrates that treatment choice hinges entirely on anatomical location, not on any difference in the organism itself.
🏥 Applied Scenario
A patient has itchy, scaling skin between their toes (tinea pedis), a child develops patchy hair loss with scalp scaling (tinea capitis), and an adult has a thickened, yellowed toenail (tinea unguium) — all potentially caused by the same organism, like Trichophyton.
Step 1
Determine treatment for the foot infection: The tinea pedis case usually clears with a topical antifungal like clotrimazole, since the infection sits at a surface level the cream can reach.
Step 2
Determine treatment for the scalp infection: The tinea capitis case is different — a topical cream alone won't work here, since the infection lives inside the hair follicle itself, so oral griseofulvin or terbinafine is required instead.
Step 3
Determine treatment for the nail infection: The tinea unguium (onychomycosis) case again requires oral terbinafine, since topical treatment can't penetrate the nail bed effectively either.
Step 4
Conclusion: All three cases could be caused by the exact same organism — the location, not the organism, is what determines both the name applied 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.
⚠️ The Trap — Assuming Any Tinea Infection Can Be Treated Topically
The most common trap is assuming any tinea infection can be treated with a topical cream, simply because most cases (pedis, corporis, cruris) respond well to that approach. Tinea capitis and tinea unguium specifically require oral antifungals, because the infection sits inside the hair follicle or nail bed where topical medication genuinely can't reach effectively, regardless of how long treatment is continued.
✓ Quick Self-Test
Answer before checking:
1. What are the three genera that cause dermatophyte (tinea) infections?
2. How is a dermatophyte infection diagnosed at the bedside?
3. Is tinea named after the organism or the body location?
4. Which two tinea presentations require oral rather than topical treatment, and why?
5. What oral medications are used for tinea capitis and tinea unguium?
Answers:
1. Trichophyton, Microsporum, and Epidermophyton.
2. A KOH prep of a skin scraping, which dissolves skin cells and reveals branching hyphae under the microscope.
3. The body location — the same organism can cause tinea pedis, capitis, cruris, corporis, or unguium depending on where it infects.
4. Tinea capitis and tinea unguium — because the infection is inside the hair follicle or nail bed, which topical medication can't penetrate.
5. Griseofulvin or terbinafine.