Dermatophytes are one of those Step 1 topics that feel “basic” until a question stem hits you with a wrestler, a kitten, a fluorescent scalp, and a KOH prep—and suddenly you’re sorting species, reservoirs, and treatments under time pressure. The good news: dermatophyte questions are extremely pattern-based. If you know what they eat (keratin), where they live (stratum corneum, hair, nails), and how they’re diagnosed (KOH + hyphae), you’ll rack up easy points.
What Are Dermatophytes?
Dermatophytes are keratinophilic fungi that infect superficial keratinized tissues:
- Skin (tinea corporis, pedis, cruris, etc.)
- Hair (tinea capitis)
- Nails (tinea unguium/onychomycosis)
They classically cause “ringworm”: annular lesions with central clearing and a scaly, erythematous advancing border.
The Big 3 Genera (memorize this)
Dermatophytes = Tinea-causing fungi:
- Trichophyton
- Epidermophyton
- Microsporum
Mnemonic: TEM (like a microscope)
First Aid cross-reference: Microbiology → Mycology → Superficial infections (Dermatophytes: Trichophyton, Microsporum, Epidermophyton)
Pathophysiology (Why They Stay Superficial)
Dermatophytes:
- Produce keratinases → digest keratin
- Invade stratum corneum (and hair/nails) but typically do not invade deeper tissues in immunocompetent patients
Why the rash looks the way it does
- The advancing edge is where the fungus is most active → erythema + scale
- The center clears as host immunity partially controls fungal growth → classic “ring” pattern
Inflammation varies by site
- Hair follicle infection (tinea capitis) can provoke strong inflammation → kerion (boggy inflammatory plaque)
- Nails have slow turnover and poor penetration of topical meds → chronic, hard-to-treat disease
Clinical Syndromes: Tinea by Location (High Yield)
Think “tinea + body part.” Here are the patterns Step questions love.
Quick table: Presentation, clues, and common therapy
| Syndrome | Where | Classic presentation | High-yield clue | Typical treatment |
|---|---|---|---|---|
| Tinea pedis | Feet | Scaling, fissures, pruritus; “moccasin” pattern | Locker rooms, occlusive shoes | Topical terbinafine/azole; oral if severe |
| Tinea cruris | Groin | Pruritic erythematous rash with scaling border; spares scrotum | “Jock itch” | Topical terbinafine/azole |
| Tinea corporis | Body | Annular plaque with central clearing | “Ringworm” | Topical terbinafine/azole |
| Tinea capitis | Scalp (kids) | Scaly patches + alopecia, “black dots” | Child, daycare; lymphadenopathy | Oral griseofulvin or terbinafine |
| Tinea unguium (onychomycosis) | Nails | Thickened, brittle, discolored nails | Recurrent, hard to eradicate | Oral terbinafine (often) |
| Tinea barbae | Beard area | Folliculitis-like lesions | Often from animals | Often oral therapy |
Step 1 pearl: Tinea capitis requires systemic therapy—topicals don’t penetrate hair shafts well.
The “Tinea Capitis” Rabbit Hole (Frequently Tested)
Endothrix vs Ectothrix hair invasion
Dermatophytes infect hair in two main patterns—this shows up as a classic board-style distinction:
- Endothrix: spores inside hair shaft → hair breaks at scalp (“black dot”)
- Commonly Trichophyton tonsurans
- Ectothrix: spores coat outside hair shaft → hair breaks above scalp
- Often Microsporum canis (zoophilic)
Wood’s lamp fluorescence (know the exception)
- Microsporum species can fluoresce blue-green under Wood’s lamp
- Trichophyton usually does not fluoresce
HY association: A child with scalp scaling + alopecia after exposure to a kitten/puppy and Wood’s lamp positive → think Microsporum canis.
First Aid cross-reference: Mycology → Superficial infections → Wood lamp: Microsporum (green fluorescence)
Diagnosis (Step-Style Approach)
1) KOH prep = cornerstone
Scrape the active border (or nail debris) and add KOH:
- KOH dissolves keratin → leaves fungal elements visible
- Dermatophytes show septate hyphae (often branching)
Common Step phrasing: “KOH prep shows branching septate hyphae.”
2) Culture (when needed)
- Sabouraud agar can culture dermatophytes
- Speciation is less common on Step than pattern recognition, but it can appear
3) Differentiate from “look-alikes”
Dermatophytes are confused with:
- Candida (intertrigo, paronychia): budding yeast, pseudohyphae; often involves mucosa; can involve scrotum
- Erythrasma (Corynebacterium minutissimum): coral-red fluorescence under Wood’s lamp
- Pityriasis rosea (herald patch): can mimic tinea corporis but KOH negative
High-yield distinction:
- Tinea cruris typically spares the scrotum
- Candida often involves the scrotum and has satellite lesions
Treatment (What Step Wants You to Pick)
First-line: allylamines and azoles
- Terbinafine (allylamine): inhibits squalene epoxidase → ↓ ergosterol, ↑ toxic squalene
- Often favored for onychomycosis and many tinea infections
- Azoles (e.g., clotrimazole, ketoconazole): inhibit 14-α-demethylase (CYP450) → ↓ ergosterol
First Aid cross-reference: Pharm → Antifungals: Terbinafine (squalene epoxidase), Azoles (14-α-demethylase)
When you MUST go systemic
- Tinea capitis: treat with oral griseofulvin or oral terbinafine
- Onychomycosis: usually needs oral terbinafine (topicals often insufficient)
Griseofulvin: still high yield
- Mechanism: disrupts microtubules → inhibits mitosis; deposits in keratin precursor cells
- Use: dermatophytes (especially tinea capitis)
- Side effects: hepatotoxicity, photosensitivity, headache; CYP450 inducer (classically tested)
High-Yield Associations & Board-Style Clues
Risk factors you’ll see in stems
- Moisture/occlusion: sweaty shoes, tight clothing
- Communal exposure: locker rooms, wrestling teams
- Animal contact: cats/dogs (Microsporum canis)
- Immunosuppression/diabetes: more extensive disease; higher recurrence
“Steroid makes it worse” = tinea incognito
Topical steroids can:
- Reduce inflammation and mask classic ring appearance
- Allow fungal spread → tinea incognito (less scaly border, atypical morphology)
Dermatophytes vs systemic dimorphic fungi (don’t mix them up)
Dermatophytes are superficial (keratinized tissue), not lung/lymph node pathogens. If the stem is pneumonia + granulomas + travel geography, you’re in dimorphic fungi territory—not dermatophytes.
Rapid Step 1 Checklist (If You Remember Nothing Else)
- Dermatophytes = Trichophyton, Microsporum, Epidermophyton (TEM)
- Infect keratinized tissues → skin, hair, nails
- KOH prep: branching septate hyphae
- Tinea capitis: oral therapy (griseofulvin or terbinafine)
- Microsporum can fluoresce green on Wood’s lamp
- Terbinafine inhibits squalene epoxidase
- Azoles inhibit 14-α-demethylase
Mini Question Prompts (Practice Your Pattern Recognition)
- Child with scaly alopecic patch + posterior cervical LAD → tinea capitis → oral griseofulvin/terbinafine
- Wrestler with annular scaly plaque on trunk → tinea corporis → topical terbinafine/azole
- Thickened yellow brittle toenail in adult → onychomycosis → oral terbinafine
- Groin rash sparing scrotum → tinea cruris (not Candida)
- Cat exposure + green fluorescence → Microsporum canis