Common Skin Rashes: Identification Guide with Key Features

Accurate rash diagnosis requires morphology, distribution, history, and sometimes biopsy. Common categories:

Inflammatory / Autoimmune

• Atopic dermatitis (eczema): Chronic, pruritic, flexural, xerosis, lichenification.
• Psoriasis: Well-demarcated erythematous plaques with silvery scale; extensor surfaces, scalp, nails.
• Lichen planus: Violaceous, polygonal, pruritic papules; Wickham striae; wrists, ankles, oral mucosa.

Acute Hypersensitivity

• Urticaria (hives): Transient wheals, pruritic, blanchable; <24h per lesion.
• Contact dermatitis: Allergic (Type IV) or irritant; geometric, at exposure site.
• Drug eruption: Morbilliform most common; onset 7–14 days post-initiation.

Infectious

• Tinea (ringworm): Annular, scaly, active border; KOH prep positive.
• Impetigo: Honey-crusted erosions; S. aureus/S. pyogenes.
• Herpes simplex/zoster: Clustered vesicles on erythematous base; dermatomal (zoster).
• Scabies: Burrows, interdigital, nocturnal itch; mite on scraping.
• Viral exanthems: Measles (cough/coryza/conjunctivitis + cephalocaudal rash), rubella, varicella (dewdrop vesicles), roseola (defervescence → rash).

Vascular / Hematologic

• Petechiae/purpura: Non-blanchable; thrombocytopenia, vasculitis, sepsis.
• Erythema nodosum: Tender nodules on shins; strep, TB, sarcoid, drugs, IBD.

When to Refer

Rapid spread, mucosal involvement, systemic signs (fever, hypotension), pain, blistering, diagnostic uncertainty.