Skin & Subcutaneous Tissue

Tinea infections: corporis, pedis, cruris, capitis

Clinical Overview and When to Suspect Tinea

— T. rubrum is the single most common dermatophyte across tinea corporis, pedis, cruris, and onychomycosis in the US

— T. tonsurans dominates tinea capitis in North American children (anthropophilic, endothrix)

— M. canis (zoophilic) from kittens/puppies causes inflammatory tinea capitis and corporis

— Annular, scaly, pruritic patch with central clearing and active raised border → tinea corporis

— Interdigital maceration or moccasin-distribution scaling on feet → tinea pedis

— Sharply demarcated erythematous rash in groin folds sparing the scrotum → tinea cruris

— Scaly patches with alopecia, broken hairs ("black dots"), occipital lymphadenopathy in a school-age child → tinea capitis

Board pearl: A patient treated empirically with a topical steroid for "eczema" who returns with a larger, less-scaly, poorly-defined lesion has tinea incognito — the steroid blunted inflammation but allowed fungal spread. Always KOH-scrape annular rashes before steroids.

Step 3 management: In ambulatory practice, confirm dermatophyte before committing to systemic antifungals (hepatotoxic, drug-interactions); topical therapy rarely needs confirmation but document failure of first-line topicals before escalation.

Tinea = superficial dermatophyte infection of keratinized tissue (stratum corneum, hair, nails) caused by Trichophyton, Microsporum, and Epidermophyton species

When to suspect in family medicine clinic:

Risk factors: humid climate, occlusive footwear, athletes (mat sports, locker rooms), diabetes, obesity, topical steroid use ("tinea incognito"), immunosuppression, shared combs/hats, household pets

Epidemiology pearl: tinea capitis is overwhelmingly a prepubertal disease; sebum's fungistatic fatty acids after puberty protect the scalp

Transmission: direct skin contact, fomites (towels, mats, hairbrushes), animal contact, or autoinoculation (tinea pedis → cruris → corporis is a classic sequence — examine feet of any patient with cruris)

Presentation Patterns and Key History

— Single or few annular, erythematous, scaly plaques on trunk/extremities with centrifugal expansion and central clearing

— Pruritus is typical but variable; lesions enlarge over weeks

— Ask about: new pets (especially kittens), wrestling/judo ("tinea gladiatorum"), gym/sauna use, prior tinea pedis

— Interdigital (most common): maceration, fissuring between 4th–5th toes

— Moccasin-type: chronic diffuse hyperkeratosis and fine scale on soles/lateral feet (often T. rubrum)

— Vesiculobullous/inflammatory: acute vesicles on instep, often T. mentagrophytes

— History clues: occlusive shoes, hyperhidrosis, communal showers, diabetes

— Erythematous, well-demarcated, half-moon plaques on medial thighs extending from inguinal fold

— Spares the scrotum (unlike candidal intertrigo, which involves scrotum and shows satellite pustules)

— Predominantly in adolescent/adult males; obesity, tight clothing, athletic activity are triggers

— Non-inflammatory ("gray patch"): scaly patch with hair loss, Microsporum

— "Black dot": broken hairs at scalp surface, T. tonsurans

— Kerion: boggy, tender, suppurative inflammatory plaque ± regional lymphadenopathy — a hypersensitivity response, not bacterial superinfection (though it can mimic abscess)

— Favus: scutula (yellow crusts), now rare

Key distinction: Tinea cruris spares scrotum; candidal intertrigo involves scrotum + satellite lesions; erythrasma fluoresces coral-red on Wood lamp; inverse psoriasis lacks scale and has sharply demarcated symmetric plaques without active border.

Board pearl: Always inspect feet and nails in any patient with tinea cruris or corporis — untreated tinea pedis/onychomycosis is the reservoir for recurrence.

Tinea corporis (ringworm of the body):

Tinea pedis (athlete's foot): three classic patterns

Tinea cruris (jock itch):

Tinea capitis: several morphologies

Key history triad to elicit at every visit: duration, prior topical treatments (especially steroids), and exposures (pets, athletes, household contacts)

Physical Exam Findings

— Annular plaque, scaly raised border, central clearing

— Border may show pustules or vesicles when actively spreading

— Majocchi granuloma: deeper follicular involvement → perifollicular papules/pustules, often after shaving over tinea or after topical steroid misuse

— Interdigital: white, soggy macerated skin between toes; check all web spaces

— Moccasin: pink, dry, fine "powdery" scale wrapping lateral feet in a slipper distribution

— Look for "two feet–one hand" syndrome: bilateral tinea pedis with unilateral tinea manuum (chronic T. rubrum)

— Check toenails for distal subungual onychomycosis (yellow, thickened, subungual debris)

— Crescentic plaques on inner thighs with sharp scaly advancing margin

— Scrotum/penis spared

— Postinflammatory hyperpigmentation common in skin of color

— Patchy alopecia with scale; broken hairs at follicle ("black dots")

— Palpate for posterior cervical/occipital lymphadenopathy — a near-mandatory finding (its absence should make you reconsider diagnosis)

— Kerion: tender, boggy, pus-draining plaque ± scarring alopecia if untreated

— Wood lamp: green fluorescence with Microsporum (ectothrix) but not T. tonsurans (endothrix, the US predominant pathogen) — Wood lamp is therefore of limited screening utility in modern US practice

Board pearl: A child with scalp scale + occipital lymphadenopathy = tinea capitis until proven otherwise; seborrheic dermatitis and atopic dermatitis do not produce regional adenopathy.

General approach: full skin exam including scalp, web spaces, soles, nails, and groin — dermatophytes love to coexist at multiple sites

Tinea corporis:

Tinea pedis:

Tinea cruris:

Tinea capitis:

Hemodynamic relevance: tinea is local; systemic signs (fever, malaise, lymphangitic streaking) should prompt evaluation for bacterial superinfection (cellulitis from interdigital tinea pedis is a classic boards pathway in diabetics/elderly)

Diagnostic Workup — Initial Office Testing

— Scrape active scaly border with #15 blade onto slide

— Apply 10–20% KOH ± DMSO; gentle heat

— Look for branching, septate hyphae crossing keratinocyte borders

— Sensitivity ~75–90% with good technique; specificity high

— Pitfalls: false negatives if patient recently used antifungal cream or steroid; scraping from central clearing instead of border

— Indicated when KOH negative but suspicion remains, when species identification matters (tinea capitis for public-health/contact tracing), or before systemic therapy in adults

— Results take 2–4 weeks

— "Comma hairs," "corkscrew hairs," and broken hairs support diagnosis

— Green-yellow fluorescence in Microsporum tinea capitis and coral-red in erythrasma (a key cruris mimic — Corynebacterium minutissimum)

— Does NOT fluoresce with T. tonsurans or T. rubrum

Step 3 management: Before starting oral terbinafine or griseofulvin, obtain KOH and/or fungal culture confirmation and document; some payers require it. For tinea capitis specifically, culture is the gold standard and guides duration if response is incomplete.

Board pearl: A KOH on scaly groin rash showing branching septate hyphae = tinea cruris; pseudohyphae + budding yeast = candida; "spaghetti and meatballs" = tinea (pityriasis) versicolor — three KOH morphologies worth memorizing.

Tinea is largely a clinical diagnosis, but confirm before committing to systemic therapy

Potassium hydroxide (KOH) prep — bedside, fast, cheap, high yield

Fungal culture (Sabouraud dextrose agar with cycloheximide/chloramphenicol)

Dermoscopy — useful adjunct for tinea capitis

Wood lamp:

PCR-based dermatophyte assays — increasingly available, rapid (days), high sensitivity; useful for onychomycosis and capitis when culture is slow

Skin biopsy with PAS stain: reserve for tinea incognito or atypical/refractory lesions where KOH is repeatedly negative

Diagnostic Workup — Advanced and Confirmatory Studies

— Sabouraud agar at 25–30°C; identification by colony morphology + microscopy

— Dermatophyte test medium (DTM) turns red in presence of dermatophytes — convenient office screen, but false positives possible if read late

— Multiplex PCR panels identify T. rubrum, T. mentagrophytes, T. tonsurans, M. canis within 1–3 days

— Particularly valuable in onychomycosis before starting 3–6 months of oral terbinafine

— Hyphae in stratum corneum confirm diagnosis when clinical and KOH disagree

— Diagnostic of choice for tinea incognito, Majocchi granuloma (hyphae within follicle/dermis)

— Terbinafine: baseline ALT/AST (FDA labeling) for any course; some clinicians omit if course <6 weeks in healthy patients, but board answer is "check LFTs"

— Itraconazole: baseline LFTs; CHF screening (negative inotrope, contraindicated in ventricular dysfunction)

— Fluconazole: baseline LFTs if extended use

— Griseofulvin: baseline LFTs and CBC for prolonged courses

— Fungal culture from scalp scale/hair using moistened cotton swab or toothbrush brushing technique

— Routine LFTs not required before standard griseofulvin or terbinafine course in healthy children per AAP, though many practitioners obtain baseline; monitor if symptoms develop

— In tinea capitis, screen siblings and household members with fungal culture; asymptomatic carriers maintain transmission

Key distinction: Onychomycosis confirmation is mandatory before months of oral therapy — PAS-stained nail clipping has the highest sensitivity (~85%), KOH ~60%, culture ~50%; combine modalities.

Board pearl: If a "psoriasis" or "eczema" plaque fails topical steroids and grows, biopsy with PAS — you are likely looking at tinea incognito or Majocchi granuloma.

Fungal culture details:

PCR / molecular assays:

Histopathology with PAS or GMS stain:

Laboratory monitoring before systemic therapy in adults:

For tinea capitis (children):

Household/contact screening:

Management Logic — Topical vs Systemic Decision

— Keratin-only (corporis, cruris, limited pedis) → topical antifungal

— Hair-bearing or nail (capitis, onychomycosis, Majocchi granuloma, extensive pedis with moccasin/vesicular pattern) → systemic required (topicals don't penetrate hair shaft or nail plate adequately)

— Tinea capitis (always)

— Tinea barbae

— Onychomycosis when treatment desired

— Extensive tinea corporis (>2–3 lesions, large body surface area)

— Failed topical therapy after 2–4 weeks

— Immunocompromised host

— Majocchi granuloma (follicular)

— Allylamines: terbinafine 1% cream BID × 1–2 weeks (fastest, fungicidal)

— Azoles: clotrimazole, miconazole, ketoconazole, econazole BID × 2–4 weeks (fungistatic)

— Ciclopirox, butenafine, naftifine as alternatives

— Tinea corporis/cruris: 2–4 weeks (continue ~1 week after clearing)

— Tinea pedis interdigital: 4 weeks

— Tinea pedis moccasin: often requires oral therapy

— Keep area dry; loose, breathable clothing

— Antifungal powder in shoes/groin for moisture control

— Launder towels/socks in hot water; do not share

— Treat household pets if zoophilic source suspected (veterinary referral)

— Treat concurrent tinea pedis when treating cruris/corporis to prevent recurrence

Step 3 management: Do NOT prescribe combination clotrimazole-betamethasone (Lotrisone) for tinea — the potent steroid causes tinea incognito, atrophy, and striae, and is a recurring wrong-answer trap. If pruritus is severe, use plain topical antifungal and a brief low-potency steroid only short-term, ideally avoided.

Board pearl: Topical nystatin has no activity against dermatophytes — it covers candida only. Choosing nystatin for tinea is a classic exam distractor.

Core principle: site of infection drives route

Indications for systemic therapy:

First-line topicals (over-the-counter and Rx):

Topical course length by site:

Adjunctive measures (always emphasize on Step 3):

Pharmacotherapy — First-Line Systemic Regimens

— Griseofulvin (microsize 20–25 mg/kg/day or ultramicrosize 10–15 mg/kg/day) PO × 6–8 weeks, taken with fatty meal to enhance absorption — historically first-line, especially for Microsporum

— Terbinafine granules 5–8 mg/kg/day × 6 weeks — preferred for T. tonsurans (the predominant US pathogen); shorter course, comparable cure, FDA-approved ≥4 years

— Adjunct: selenium sulfide 2.5% or ketoconazole 2% shampoo 2–3×/week for the patient and household members to reduce shedding/transmission

— Treat kerion with same oral antifungal ± short prednisone taper for severe inflammation; incision and drainage is wrong (it's hypersensitivity, not abscess)

— Terbinafine 250 mg PO daily × 6 weeks (fingernails) or 12 weeks (toenails) — first-line, ~70% mycologic cure

— Itraconazole pulse therapy alternative

— Terbinafine 250 mg PO daily × 2–4 weeks, or

— Itraconazole 200 mg PO daily × 1 week, or

— Fluconazole 150–200 mg PO weekly × 2–4 weeks

— Terbinafine: inhibits CYP2D6 (caution with TCAs, beta-blockers, SSRIs); hepatotoxicity, dysgeusia, rare neutropenia

— Itraconazole: potent CYP3A4 inhibitor — DO NOT combine with statins (rhabdomyolysis), warfarin, many tyrosine kinase inhibitors; negative inotrope — avoid in HFrEF

— Fluconazole: QT prolongation, hepatotoxicity, CYP2C9/3A4 interactions

— Griseofulvin: reduces OCP efficacy — counsel on backup contraception; teratogen — avoid pregnancy during and 1 month after

Step 3 management: For a teenage girl on combined OCPs requiring oral antifungal for tinea capitis, terbinafine is preferred over griseofulvin to avoid contraceptive failure and teratogenicity risk.

Board pearl: Terbinafine is fungicidal; azoles are fungistatic — terbinafine therefore gives shorter courses and higher cure rates for dermatophytes.

Tinea capitis (children > adults; always systemic):

Onychomycosis (adults):

Extensive tinea corporis/cruris/pedis:

Majocchi granuloma: systemic terbinafine × 4–6 weeks

Key drug interactions and cautions:

Expanded Pharmacology and Special Treatment Scenarios

— Stop topical steroid abruptly → rebound flare warns patient

— Confirm with KOH/biopsy; treat with oral terbinafine 250 mg daily × 2–4 weeks because barrier is disrupted and follicular involvement common

— Topicals fail because hyphae are intrafollicular/dermal

— Oral terbinafine × 4–6 weeks; consider biopsy to confirm

— Stop ongoing shaving over lesion and discontinue any topical steroid

— Highly transmissible T. tonsurans on head/neck/arms

— Treat with oral terbinafine or itraconazole; topical alone often insufficient

— Return to play: NCAA/state athletic association rules generally require 72 hours of systemic therapy (and lesion coverage) before competition; scalp lesions require 2 weeks of oral therapy

— Oral terbinafine 250 mg × 2 weeks

— For vesicular: Burow solution (aluminum acetate) soaks to dry, then topical antifungal

— Efinaconazole 10% solution, tavaborole 5% solution, ciclopirox 8% lacquer — for mild/moderate, ≤50% nail involvement; cures <20%

— Laser therapy: cosmetic improvement only, not curative

— Lower threshold for oral therapy

— Longer courses (often 2× standard)

— Watch for deep dermatophytosis

— Aggressive treatment + foot care education to prevent bacterial cellulitis through fissures — a Step 3 favorite causal chain

CCS pearl: When managing tinea capitis in clinic, your orders are: KOH/fungal culture → start oral terbinafine OR griseofulvin → selenium sulfide shampoo for patient AND household → educate on no sharing combs/hats → follow-up in 4–6 weeks with repeat culture to assess clearance before stopping therapy.

Board pearl: Itraconazole + simvastatin = rhabdomyolysis trap; switch statin or use terbinafine.

Tinea incognito (steroid-modified tinea):

Majocchi granuloma:

Tinea gladiatorum (wrestlers):

Refractory tinea pedis (moccasin or vesicular):

Onychomycosis non-oral options (limited efficacy):

Immunocompromised patients (HIV, transplant, biologics):

Diabetic patients with tinea pedis:

Special Populations — Elderly and Hepatic/Renal Impairment

— Onychomycosis prevalence rises to 30–50% over age 70 — but treatment is elective in many cases; weigh hepatotoxic risk vs cosmetic/functional benefit

— Treat when: diabetic foot risk, pain/discomfort, recurrent cellulitis, immunosuppression

— Polypharmacy increases interaction risk — review medication list before any oral azole

— Tinea pedis is a major reservoir for lower-extremity cellulitis in elderly; aggressive topical therapy + interdigital care is high-yield prevention

— Terbinafine: contraindicated in chronic or active liver disease; rare idiosyncratic hepatic failure

— Itraconazole, ketoconazole, fluconazole: all hepatotoxic; oral ketoconazole is no longer recommended for dermatophytes due to severe hepatotoxicity and adrenal suppression (FDA boxed warning)

— Griseofulvin: contraindicated in severe hepatic dysfunction and porphyria

— In cirrhosis or significant LFT elevation: prefer topical therapy wherever feasible; if oral required, lower threshold for monitoring

— Terbinafine: not recommended if CrCl <50 mL/min per labeling (limited pharmacokinetic data; metabolite accumulation)

— Fluconazole: dose-reduce by 50% when CrCl <50; dialyzable

— Itraconazole oral solution: avoid in CrCl <30 (cyclodextrin vehicle accumulates)

— Griseofulvin: no renal dose adjustment needed

— Baseline LFTs for terbinafine and azoles; recheck at 4–6 weeks for prolonged courses

— Discontinue immediately for symptoms: anorexia, nausea, RUQ pain, jaundice, dark urine

Key distinction: In an elderly diabetic with tinea pedis and recurrent cellulitis, treating the tinea is secondary prevention for cellulitis — a guideline-endorsed indication that often outweighs hepatotoxicity concerns. This is a high-yield Step 3 reasoning chain.

Step 3 management: For an 80-year-old with onychomycosis and no functional complaint, shared decision-making favors no treatment or topical efinaconazole rather than 3 months of oral terbinafine — emphasize value-based, low-harm care.

Elderly:

Hepatic impairment:

Renal impairment:

Monitoring:

Special Populations — Pregnancy, Pediatrics, and Athletes

— Topical therapy is preferred: clotrimazole, miconazole, ciclopirox are considered safe (FDA category B historically)

— Topical terbinafine: limited data, generally acceptable for small areas

— Oral antifungals — avoid:

— Griseofulvin: teratogenic (animal data, conjoined twins reported); avoid in pregnancy and for 1 month after in women, 6 months after in men (sperm)

— Oral terbinafine: category B but limited human data — defer treatment of onychomycosis until postpartum

— Fluconazole: high-dose (>400 mg/day) associated with craniofacial anomalies; single low-dose for candidiasis acceptable but routinely avoid for dermatophytes

— Itraconazole: teratogenic, contraindicated

— Topicals safe; avoid application to nipple

— Oral terbinafine excreted in milk — avoid

— Tinea capitis is overwhelmingly pediatric (age 3–10)

— Griseofulvin FDA-approved >2 years; terbinafine granules FDA-approved ≥4 years

— Topicals alone are inadequate for capitis — always systemic

— Counsel families: no school exclusion required for tinea capitis once treatment started and lesions covered (per AAP Red Book); for tinea corporis on exposed skin, cover lesion

— Screen household contacts; treat symptomatic siblings; sporicidal shampoo for asymptomatic carriers

— Daily skin checks; cover lesions

— Return-to-competition rules: ≥72 hours of systemic antifungal + lesion covered for skin tinea; 14 days of oral therapy for tinea capitis

— Disinfect mats with appropriate sporicidal solutions

Board pearl: Pregnant patient with bothersome tinea cruris → topical clotrimazole, NOT oral fluconazole, and NOT a combination steroid product.

Step 3 management: Child with tinea capitis: oral terbinafine OR griseofulvin + selenium sulfide shampoo for child AND siblings/parents (sporicidal adjuvant) + return to school once treatment initiated.

Pregnancy:

Breastfeeding:

Pediatrics:

Athletes (wrestlers, gymnasts, mat sports):

Complications and Adverse Outcomes

— Cellulitis from interdigital tinea pedis fissures — classic in diabetics, lymphedema, elderly; Streptococcus pyogenes and S. aureus

— Treating tinea pedis reduces recurrent lower-extremity cellulitis (number needed to treat is meaningful in high-risk patients)

— Erysipelas with sharp-bordered facial or leg erythema may follow occult tinea pedis

— Follicular rupture → deeper dermatophyte infection with perifollicular nodules

— Often triggered by topical steroid use or shaving (women shaving legs over corporis lesion is classic vignette)

— Long-term topical steroid → atrophy, telangiectasias, striae, and persistent atypical fungal infection

— Untreated tinea capitis kerion can leave permanent cicatricial alopecia

— Early oral antifungal ± brief steroid taper prevents this

— Sterile, pruritic vesicular eruption on hands or trunk in response to active tinea elsewhere (often inflammatory tinea pedis)

— Treat the underlying tinea; id reaction resolves with primary disease control

— Do NOT treat the id reaction with antifungals topically — it contains no fungus

— Nail dystrophy, pain, ingrown nails, recurrent paronychia, footwear difficulty

— Functional impairment in elderly → falls risk

— Terbinafine: hepatotoxicity, taste disturbance (may persist months), neutropenia

— Itraconazole: heart failure exacerbation

— Griseofulvin: photosensitivity, headache, OCP failure

Key distinction: Id reaction = distant sterile vesicles, KOH-negative, resolves with treatment of primary tinea. Don't biopsy or culture the hand vesicles — examine the feet.

Board pearl: Recurrent unilateral leg cellulitis without obvious portal of entry → examine interdigital web spaces; chronic tinea pedis is the missed source.

Bacterial superinfection:

Majocchi granuloma:

Tinea incognito:

Kerion and scarring alopecia:

Id reaction (dermatophytid):

Onychomycosis sequelae:

Drug-related complications:

Psychosocial: stigma in athletes, school children; missed school/practice

When to Escalate Care — Consult, Inpatient, or Specialist Referral

— Diagnostic uncertainty after KOH/culture

— Tinea incognito or Majocchi granuloma requiring biopsy

— Refractory infection despite appropriate systemic therapy

— Extensive or recurrent disease in immunocompromised host

— Severe kerion or scarring alopecia risk

— Recurrent or treatment-failure tinea capitis

— Outbreaks in schools, daycares, or athletic teams

— Atypical organisms (e.g., T. soudanense in returned travelers)

— Cellulitis with systemic signs complicating tinea pedis → IV antibiotics, admission per usual cellulitis criteria

— Severe inflammatory kerion with systemic symptoms in a child → consider admission for IV therapy if oral intake compromised; ID consult

— Disseminated or deep dermatophytosis in immunocompromised → biopsy, systemic therapy, ID consult

— Tinea capitis outbreaks in schools/daycares → notify school nurse; institute screening

— Wrestling team outbreaks → coach, athletic trainer, team physician coordination

— Zoophilic source → veterinary evaluation of pet

— Multiple drug interactions (HIV ART, transplant immunosuppression, anticoagulation) before initiating oral azoles

CCS pearl: A 6-year-old presents with a fluctuant scalp mass and lymphadenopathy after weeks of "shampoo treatment." Orders: KOH and culture → oral griseofulvin or terbinafine → consider short prednisone if severe inflammation → do NOT incise and drain (kerion is hypersensitivity, not abscess) → dermatology referral if no improvement at 4 weeks → school notification.

Step 3 management: Hospital admission is rarely needed for tinea itself; admit for complications (cellulitis with sepsis, severe kerion in inability to eat) — keep this clean on the exam.

Board pearl: Incising a kerion is a classic wrong answer — antifungals, not the scalpel, are the cure.

Dermatology referral (outpatient):

Pediatric dermatology / infectious disease:

Inpatient / urgent escalation — uncommon but exam-relevant:

Public health considerations:

Pharmacy consultation:

Key Differentials — Other Superficial Skin Conditions

— Coin-shaped, intensely pruritic plaques on extremities

— No central clearing, uniformly inflamed/scaly throughout

— Often bilateral, symmetric; KOH negative; responds to topical steroids

— Well-demarcated, bright red, sharply defined plaques with silvery scale on extensor surfaces

— Inverse psoriasis in groin: smooth, shiny, no scale; bilateral symmetric, involves gluteal cleft

— Nail pitting, family history

— Greasy yellow scale, diffuse, no patchy alopecia, no lymphadenopathy, all ages

— Flexural distribution, lichenification, intense pruritus, atopic history

— Often misdiagnosed as tinea or vice versa in dark skin

— Herald patch then "Christmas tree" distribution on trunk

— Lesions are oval with peripheral collarette of scale; self-limited 6–8 weeks

— Annular ring of firm dermal papules, no scale, no pruritus

— KOH negative; often dorsal hands/feet

— Annular photodistributed plaques; ANA/anti-Ro positive; KOH negative

— Single expanding non-scaly erythematous patch, often with central clearing — but history of tick exposure in endemic area

— Hypo- or hyperpigmented macules on trunk; KOH shows "spaghetti and meatballs" (Malassezia) — not a true dermatophyte

Key distinction: Scale + annular + active border + central clearing + KOH-positive = tinea. Scale absent or universal across lesion, KOH negative = mimic.

Board pearl: A solitary annular lesion called "ringworm" that's been "spreading for a month despite antifungal" should make you reconsider — top differentials are granuloma annulare, subacute cutaneous lupus, and nummular eczema.

Nummular eczema:

Psoriasis (especially inverse psoriasis for cruris):

Seborrheic dermatitis (mimics tinea capitis):

Atopic dermatitis:

Pityriasis rosea:

Granuloma annulare:

Subacute cutaneous lupus:

Erythema migrans (Lyme):

Pityriasis versicolor:

Key Differentials — Other Categories (Cruris/Pedis Mimics)

— Beefy red plaques with satellite pustules, scrotum/penis involved

— Moist macerated folds; common in diabetes, obesity, incontinence

— KOH: pseudohyphae and budding yeast

— Treat: topical nystatin OR azole + moisture control

— Corynebacterium minutissimum, gram-positive rod

— Sharply demarcated brown-red patches in folds, minimal scale

— Wood lamp: coral-red fluorescence (porphyrin)

— Treat: topical clindamycin or erythromycin, oral erythromycin for extensive

— Symmetric, bright red, non-scaly plaques in folds including gluteal cleft

— Treat: low-potency topical steroid, calcineurin inhibitor

— Match clothing or product exposure pattern; intensely pruritic; vesicles possible

— Symmetric pruritic deep-seated vesicles ("tapioca pudding") on lateral fingers/toes/palms/soles

— KOH negative; treat with topical steroid

— Punched-out pits on weight-bearing soles; foul odor; sweaty feet

— Kytococcus sedentarius (bacterial); topical erythromycin/clindamycin

— Distribution matches shoe (dorsum of foot more than soles); KOH negative

— Burrows, web spaces, genitals, nocturnal pruritus

— Persistent "eczematous" patches/plaques unresponsive to therapy; biopsy

Key distinction: Scrotum involved → think candida; scrotum spared → think tinea cruris. Coral-red Wood lamp → erythrasma, not tinea.

Board pearl: A football player with pitted, malodorous, sweaty soles is pitted keratolysis (bacterial) — antifungals will fail. Treat with topical antibiotic and antiperspirant.

Candidal intertrigo (cruris mimic):

Erythrasma (cruris mimic):

Inverse psoriasis (cruris mimic):

Contact dermatitis (cruris/corporis mimic):

Dyshidrotic eczema (pedis mimic):

Pitted keratolysis (pedis mimic):

Allergic/irritant contact dermatitis to footwear (pedis mimic):

Scabies (corporis mimic in some presentations):

Cutaneous lymphoma (mycosis fungoides):

Secondary Prevention and Long-Term Plan

— Dry skin thoroughly after bathing, especially interdigital web spaces and groin folds

— Wear cotton or moisture-wicking socks; change daily; alternate shoes day-to-day to allow drying

— Antifungal powders (miconazole, tolnaftate) in shoes and groin for prone patients

— Shower shoes in communal showers, locker rooms, pool decks

— Do not share towels, combs, hats, razors, athletic gear

— Hot-water launder bedding and clothing; consider replacing/disinfecting old shoes

— Concurrent tinea pedis or onychomycosis must be addressed to prevent corporis/cruris recurrence

— Treat household contacts and pets when zoophilic source suspected (kittens, puppies, guinea pigs)

— Pre-season skin checks

— Mat sanitation, no shared gear

— Prophylactic fluconazole has been used in some wrestling programs but not guideline-endorsed

— Daily foot inspection — part of standard ADA foot-care counseling

— Aggressive treatment of tinea pedis as cellulitis prevention

— Annual comprehensive foot exam by primary care or podiatry

— Continue sporicidal shampoo (selenium sulfide, ketoconazole) 2–3×/week for 2–4 weeks after clearing to reduce reinfection

— Replace combs, brushes, hats; launder pillowcases

— Recurrence ~25% within 2–3 years; counsel on long-term shoe/sock hygiene and topical antifungal powder maintenance

Step 3 management: For a patient on their third episode of tinea cruris in a year, the right answer set includes simultaneous treatment of tinea pedis, daily antifungal powder in groin, loose cotton underwear, and weight-loss counseling if applicable — not stronger antifungal.

Board pearl: Recurrence after successful treatment is usually a reservoir problem (feet, nails, household) or a moisture problem — address both.

Hygiene and environmental measures (cornerstone of prevention):

Treat reservoirs:

Athlete-specific prevention:

Diabetic/immunocompromised patients:

Tinea capitis post-treatment:

Onychomycosis:

Follow-Up, Monitoring, and Counseling

— Reassess at 2–4 weeks; expect symptomatic improvement within 1 week

— Continue topical therapy 1 week beyond clinical resolution to ensure mycologic cure

— If no improvement at 4 weeks: reconsider diagnosis, obtain KOH/culture, consider systemic therapy

— Clinical check at 4–6 weeks

— Baseline LFTs; recheck if symptoms or prolonged course

— Counsel on dysgeusia (taste loss, may persist), GI upset, rare hepatotoxicity

— Follow-up at 3 and 6 months — nail growth is slow; clinical clearance lags treatment by months

— Mycologic cure (negative KOH + culture) at 6–12 months is the goal

— Photograph at baseline to track progress objectively

— Follow-up at 4–6 weeks with repeat scalp KOH ± culture

— Extend therapy 2–4 more weeks if culture still positive

— Hair regrowth occurs over 2–3 months

— Family education: not contagious once treated 1–2 weeks, but contact tracing matters

— Diagnosis and expected course

— Medication adherence importance (premature discontinuation → relapse)

— Hygiene, footwear, shared item avoidance

— When to return: worsening, signs of cellulitis (fever, expanding red streaks), kerion development, adverse drug effects

— Cost considerations: terbinafine generic is inexpensive; itraconazole and topical lacquers may be costly — discuss before prescribing

— Avoid unnecessary oral antifungals for limited disease (overprescribing leads to LFT surveillance, drug interactions, cost)

— Avoid Lotrisone (clotrimazole-betamethasone) — high cost, harm, and dermatologic society discouragement

CCS pearl: On every tinea visit close, document: diagnosis confirmation method, prescribed regimen, return precautions (cellulitis signs, no improvement at 2–4 weeks), and follow-up appointment. Closing the loop is a Step 3 quality measure.

Tinea corporis / cruris / pedis (topical therapy):

Systemic terbinafine for corporis/pedis:

Onychomycosis:

Tinea capitis:

Counseling points to document:

Quality / value-based care:

Ethical, Legal, and Patient Safety Considerations

— Discuss hepatotoxicity risk with terbinafine and azoles; document

— For onychomycosis (largely cosmetic), the risk-benefit conversation is especially important — patients should understand cure rates (~70%), recurrence (~25%), and monitoring requirements

— In elderly with polypharmacy, shared decision-making is the standard

— Reconcile all medications before starting itraconazole (CYP3A4) or fluconazole — statins, anticoagulants, antiarrhythmics, immunosuppressants, HIV therapy

— Pharmacist consultation is a recognized patient-safety intervention

— Griseofulvin reduces OCP efficacy — counsel on backup contraception, document

— Griseofulvin contraindicated in pregnancy; men should not father children for 6 months after treatment (label-based)

— Pregnancy testing before initiating oral antifungals in reproductive-age women is reasonable

— Tinea capitis outbreaks in schools/daycares often trigger notification to school nurse and parents of contacts; not nationally reportable but state and local guidance applies

— Athletic outbreaks (wrestling): report per state athletic association rules

— Patients discharged on 3-month terbinafine course for onychomycosis must have a clear primary care follow-up plan for LFT monitoring and reassessment — a classic dropped-ball scenario

— Hospitalists prescribing oral antifungals for incidentally noted disease should communicate plan to PCP

— Don't prescribe combination clotrimazole-betamethasone — patient-safety event in itself due to steroid-induced tinea incognito, atrophy, striae; multiple dermatology societies recommend against

— Don't incise a kerion (avoidable scarring alopecia)

— Don't use oral ketoconazole for dermatophytes — FDA boxed warning for hepatotoxicity and adrenal insufficiency

Step 3 management: A reproductive-age woman on combined OCP requiring oral antifungal — counsel on backup contraception, prefer terbinafine over griseofulvin, document the counseling. This is a recurring vignette.

Board pearl: Combination steroid-antifungal creams are a patient-safety red flag on Step 3; the correct answer is almost always a single-agent antifungal.

Informed consent for oral antifungals:

Drug interaction safety:

Pregnancy and contraception counseling:

Mandatory reporting and public health:

Transition-of-care risk:

Avoidance of harm:

High-Yield Associations and Rapid-Fire Facts

— Branching septate hyphae → dermatophyte

— Pseudohyphae + budding yeast → candida

— Spaghetti and meatballs → pityriasis (tinea) versicolor (Malassezia)

— Terbinafine: fungicidal, CYP2D6 inhibitor, dysgeusia, hepatotoxic

— Itraconazole: negative inotrope (avoid in HFrEF), CYP3A4 inhibitor

— Fluconazole: QT, hepatotoxicity, weekly dosing option

— Griseofulvin: take with fatty meal, reduces OCP, teratogen

— Oral ketoconazole: avoid for dermatophytes (FDA warning)

Board pearl: Nystatin covers candida only — no activity against dermatophytes. This is one of the most repeated traps in dermatology vignettes.

Most common dermatophyte overall in US: Trichophyton rubrum

Most common cause of tinea capitis in US: T. tonsurans (endothrix, no Wood-lamp fluorescence)

Zoophilic dermatophytes (kittens, puppies): M. canis — green fluorescence, more inflammatory

"Two feet–one hand" syndrome: bilateral tinea pedis + unilateral tinea manuum (dominant hand, from scratching/picking feet) → chronic T. rubrum

Tinea cruris spares the scrotum; candidal intertrigo involves the scrotum with satellite lesions

Erythrasma: coral-red Wood lamp fluorescence; Corynebacterium minutissimum; treat with topical/oral erythromycin or clindamycin

Kerion: hypersensitivity reaction, NOT abscess — do not I&D; treat with oral antifungal ± brief steroid

Majocchi granuloma: follicular dermatophytosis from steroid use or shaving; requires oral terbinafine

Id reaction: sterile vesicular eruption distant from primary infection; treat the primary site

Tinea incognito: steroid-modified tinea, loss of typical features

KOH morphologies:

Onychomycosis confirmation: PAS-stained nail clipping has highest sensitivity

Drugs and pearls:

Tinea gladiatorum return-to-play: 72 hours systemic therapy for skin tinea; 14 days for capitis

AAP Red Book: tinea capitis treated children may attend school

Recurrent leg cellulitis in adults: examine the feet/web spaces — chronic tinea pedis is the portal

Board Question Stem Patterns

Step 3 management: Reading vignettes, anchor on these question keys: location of rash, scrotum/labia involvement, prior steroid use, exposures, drug interactions, and pregnancy status — they almost always pick the answer.

"6-year-old with scaly scalp patch and tender posterior cervical nodes" → tinea capitis, oral griseofulvin or terbinafine (not topical alone); KOH/culture before therapy; selenium sulfide shampoo as adjuvant

"Wrestler with annular plaques on neck and arms" → tinea gladiatorum, T. tonsurans; oral terbinafine; 72-hour rule before return to mat

"Diabetic with recurrent right leg cellulitis" → examine interdigital spaces → chronic tinea pedis as portal of entry; treat the fungus to prevent recurrence

"Annular rash treated with hydrocortisone became larger and less scaly" → tinea incognito; KOH or biopsy, then oral terbinafine

"Pregnant patient with itchy groin rash, scrotum/labia spared" → tinea cruris in pregnancy → topical clotrimazole or miconazole, avoid oral antifungals

"Boggy fluctuant scalp mass in child with patchy hair loss" → kerion → oral antifungal ± brief prednisone; DO NOT incise and drain

"Athlete with foul-smelling pitted soles" → pitted keratolysis (bacterial), not tinea; topical erythromycin/clindamycin

"Beefy red groin rash with scrotal involvement and satellite pustules" → candidal intertrigo, not tinea cruris; topical nystatin or azole

"Coral-red fluorescence in groin" → erythrasma, topical clindamycin/erythromycin

"Patient on simvastatin develops tinea — which oral antifungal?" → terbinafine (itraconazole/fluconazole are CYP3A4 inhibitors → rhabdomyolysis)

"Woman shaving legs develops perifollicular papules within an annular plaque" → Majocchi granuloma; oral terbinafine

"Adolescent on combined OCP needs oral antifungal" → terbinafine preferred over griseofulvin (no OCP interaction)

"Patient with HFrEF EF 25% needs systemic antifungal for onychomycosis" → avoid itraconazole (negative inotrope); use terbinafine

"Itchy vesicles on hands appear after starting treatment for inflammatory tinea pedis" → id reaction; continue treating the feet

One-Line Recap

High-yield recap bullets:

— Avoid clotrimazole-betamethasone (tinea incognito)

— Avoid oral ketoconazole for dermatophytes (FDA warning)

— Itraconazole contraindicated in HFrEF and CYP3A4-vulnerable regimens (statins)

— Griseofulvin reduces OCP efficacy and is teratogenic

— Nystatin does not treat dermatophytes

Board pearl: If a Step 3 vignette mentions "ringworm," "expanding annular," "central clearing," "wrestler," or "kitten" — the diagnosis is almost certainly tinea; the test point is which drug, which route, and which patient-safety pitfall you avoid.

Tinea is a clinical-and-KOH diagnosis treated topically when limited to keratin (corporis, cruris, mild pedis) and systemically when hair or nail is involved (capitis, onychomycosis, Majocchi granuloma) — with terbinafine the workhorse oral drug and combination steroid-antifungal creams a recurring patient-safety trap.

Site dictates route: topical for skin-only; oral for capitis (always), onychomycosis, Majocchi, extensive/refractory disease

Confirm before going systemic: KOH at the active border; PAS-stained nail clipping for onychomycosis; fungal culture for capitis

Treat reservoirs and contacts: address tinea pedis/nails in recurrent cruris; selenium sulfide shampoo for capitis patient AND household; treat zoophilic pet sources

Know the drug traps:

Don't I&D a kerion — it is a hypersensitivity reaction, not an abscess; oral antifungal ± brief prednisone prevents scarring alopecia

Tinea cruris spares scrotum; candida involves scrotum + satellite lesions; erythrasma fluoresces coral-red

Examine feet of every patient with corporis, cruris, or recurrent lower-extremity cellulitis — chronic tinea pedis is the missed reservoir

Pregnancy: topical azoles only; defer onychomycosis treatment