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Describe the lesion
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Here is a detailed description of the lesion:
Lesion Description
Location: Extensor surface of the upper arm, overlying the posterior elbow/proximal forearm region.
Morphology: A solitary, well-demarcated annular (ring-shaped) plaque - the classic "ringworm" appearance.
Border: Clearly elevated, erythematous (pink-red) outer rim with a thin collarette of fine scaling tracing the inner margin of the raised edge. The border has a clean, circular outline.
Center: Slightly depressed and paler than the border, with a tiny central darker pinpoint crust - consistent with central clearing, a hallmark feature.
Surface: Dry, with superficial fine scaling concentrated along the active border. No surrounding erythema, pustules, or significant induration of the surrounding skin.
Size: Approximately 1.5-2 cm in diameter based on proportions.
Most Likely Diagnosis: Tinea Corporis (Ringworm)
This presentation is textbook tinea corporis - a dermatophyte (fungal) infection of glabrous skin. As described in Fitzpatrick's Dermatology, "the classic presentation is that of an annular ('ringworm'-like) or serpiginous plaque with scale across the entire active erythematous border. The border...advances centrifugally. The center of the plaque is usually scaly but may exhibit complete clearance."
Differential Diagnoses to Consider
| Condition | Distinguishing Features |
|---|---|
| Tinea corporis (most likely) | Active scaly border, central clearing, annular |
| Nummular eczema | More vesicular, oozing; less sharp borders |
| Psoriasis (plaque) | Thicker silvery scale; extensor surfaces but usually symmetric/multiple |
| Granuloma annulare | No scale, skin-colored to red papules forming a ring |
| Pityriasis rosea (herald patch) | Collarette scale, trunk > limb |
Recommended Next Steps
- KOH preparation - scrape the active scaly border and examine under microscopy for fungal hyphae (confirms diagnosis).
- Topical antifungal treatment: clotrimazole 1% or terbinafine 1% cream applied twice daily for 2-4 weeks is effective for a single lesion.
- Avoid topical steroids alone - they can suppress inflammation without clearing the infection and may produce "tinea incognito."
Note: This is a visual/clinical assessment only and is not a substitute for in-person evaluation and laboratory confirmation by a healthcare provider.
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