The Annular Lesion — Ringworm on Physical Exam
Placeholder: classic ring-shaped lesion, active border, clearing center.
- Annular: Shape (ring with central clearing)
- Raised, scaly: Border (active leading edge)
- Centrifugal: Growth (expands outward over weeks)
- Trunk, limbs: Common sites (exposed, warm-moist skin)
What the lesion looks like
Placeholder text: ring-shaped plaque, scaly active edge, clear healing center.
Why the border stays active
Placeholder text: fungus spreads outward, immune response clears the center.
Wood's Lamp — Fast Screen, Limited Sensitivity
Placeholder: UV light can help, but many dermatophytes stay dark.
- ~365 nm: Wavelength (long-wave UV-A lamp)
- Fluoresce: Microsporum spp. (apple-green glow)
- Usually dark: Trichophyton spp. (most common cause, no glow)
- Low: Overall sensitivity (cannot rule out tinea alone)
How the test works
Placeholder text: darkened room, UV lamp held near lesion, glow noted.
Why it can mislead
Placeholder text: negative glow does not exclude tinea corporis.
KOH Prep — The Definitive Bedside Test
Placeholder: scraping dissolved in KOH reveals hyphae under the scope.
- Active border: Sample site (highest fungal density)
- 10–20% KOH: Reagent (dissolves keratin, not hyphae)
- Septate hyphae: Positive finding (branching, uniform width)
- Minutes: Turnaround (bedside microscope read)
Collecting the sample
Placeholder text: scalpel scrapes scale from the active advancing edge.
Reading the slide
Placeholder text: branching septate hyphae cross skin cell borders.
Ringworm vs. Nummular Eczema
Placeholder: eczema mimics ringworm but fails every fungal test.
- Uniform patch: Eczema shape (no central clearing)
- Not raised: Eczema border (diffuse, ill-defined edge)
- Negative: Eczema Wood's lamp (no fluorescence ever)
- Negative: Eczema KOH (no hyphae, only skin cells)
Key visual differences
Placeholder text: coin-shaped uniform plaques, itchy, symmetric distribution.
Why testing still matters
Placeholder text: overlap in appearance makes KOH prep essential.
Confirmed Diagnosis Guides Treatment
Placeholder: positive hyphae confirm tinea, steer therapy correctly.
- KOH positive: Confirmatory test (septate branching hyphae)
- Topical antifungal: Correct treatment (terbinafine, clotrimazole)
- Topical steroids: Avoid (can worsen tinea (incognito))
- Emollients/steroids: Eczema treatment (if KOH negative)
From microscope to prescription
Placeholder text: hyphae seen, diagnosis confirmed, antifungal started.
Why the distinction matters
Placeholder text: steroids on tinea can mask and worsen the fungus.
Placeholder highlight: never treat an undiagnosed ring with steroids alone.