Dermatophytes are a classic “easy points” topic on USMLE—until the names start blending together. The trick is to lock in which genera cause tinea, what they eat, and what clinical/lab clues distinguish them. Here’s a quick, shareable mnemonic that makes dermatophytes stick.
The Big Picture (what you must know)
Dermatophytes are molds that infect keratinized tissues:
- Skin (tinea corporis, pedis, cruris)
- Hair (tinea capitis)
- Nails (onychomycosis = tinea unguium)
They’re typically superficial, and rarely invade deeper tissues in immunocompetent patients because they feed on keratin.
The Core Mnemonic: “M.E.T. eats KeratIn”
M.E.T. = the dermatophyte genera
- Microsporum
- Epidermophyton
- Trichophyton
One-liner:
“M.E.T. eats keratIn” → Dermatophytes are Microsporum, Epidermophyton, Trichophyton, and they digest keratin (skin/hair/nails).
Visual Hook (mental image)
Picture a subway turnstile labeled “M.E.T.”
A person tries to enter holding a giant bag labeled KERATIN (skin, hair, nails).
The M.E.T. gate “accepts” only keratin—because dermatophytes love keratinized tissue.
High-Yield Differentiators (Step 1 gold)
Memorize what each genus likes to infect:
| Genus | Skin | Hair | Nails | Classic Step Clue |
|---|---|---|---|---|
| Microsporum | Yes | Yes | Sometimes | Fluoresces (some species) under Wood lamp |
| Epidermophyton | Yes | No | Yes | No hair involvement |
| Trichophyton | Yes | Yes | Yes | Most common cause overall (esp. tinea pedis/onychomycosis) |
Mini-mnemonic inside the mnemonic:
- Epidermophyton = Epidermis + nails (NO hair)
- Microsporum = think “M” for “Many hairs” (hair involvement common)
USMLE-Style Clinical Anchors (rapid recall)
Common tinea patterns
- Tinea pedis (“athlete’s foot”): interdigital scaling, moccasin distribution
- Tinea cruris (“jock itch”): pruritic groin rash, often spares scrotum
- Tinea corporis: annular “ringworm,” advancing scaly border + central clearing
- Tinea capitis: alopecia patches + scale; can form kerion (boggy inflammatory mass)
- Onychomycosis: thickened, discolored, brittle nails
Lab + Diagnosis: what test do they keep asking about?
KOH prep
- KOH dissolves keratin → lets you see fungal elements
- Dermatophytes show septate hyphae
Culture (classic)
- Sabouraud agar (fungal culture medium)
Wood lamp (select species)
- Some dermatophytes (not all) fluoresce:
- Microsporum canis is a common fluorescent association
- Don’t overgeneralize: Wood lamp negativity doesn’t rule out tinea.
Treatment (high yield and practical)
Superficial skin (most tinea corporis/cruris/pedis)
- Topical terbinafine or topical azole (e.g., clotrimazole)
Nails or hair (onychomycosis, tinea capitis)
- Needs systemic therapy (topicals don’t penetrate well)
- Terbinafine (common first-line for onychomycosis)
- Griseofulvin (classic board favorite for tinea capitis; also used clinically)
- High-yield mechanism vibe: deposits in keratin, inhibits fungal mitosis (microtubules)
Clinical pearl:
If it’s in the hair follicle or nail plate, think systemic.
Quick Share “Flashcard” Summary
- Dermatophytes = M.E.T.: Microsporum, Epidermophyton, Trichophyton
- They eat keratin → infect skin, hair, nails
- Epidermophyton: no hair
- Dx: KOH = septate hyphae
- Tx: topical for skin; systemic for hair/nails