Key Concept: Tinea corporis = annular, SCALY, raised border with central clearing. Distinguish from erythema migrans (not scaly, tick exposure) and psoriasis (symmetrical extensor involvement).
Diagnosis
Key Features
Tinea Corporis
Annular lesion with raised, scaly border + central clearing
Pruritic, starts as small spot and grows centrifugally
KOH prep shows hyphae; caused by Trichophyton rubrum
Treatment: topical clotrimazole or terbinafine (localized); oral terbinafine (extensive)
Erythema Migrans
Bull's-eye appearance; often NOT scaly, NOT raised border
History of tick bite in endemic area; may have flu-like symptoms
Treatment: oral doxycycline (Lyme disease)
Psoriasis
Symmetric scaly plaques on extensor surfaces (elbows, knees)
Silvery scale on erythematous base; Auspitz sign (pinpoint bleeding when scale removed)
Associated with psoriatic arthritis, nail pitting
Urticaria
Pruritic wheals that resolve within 24 hours; no scale
Triggered by allergen/IgE-mediated reaction; associated angioedema
Treatment: antihistamines; epinephrine if anaphylaxis