Dermatology
Core concepts
Learn primary lesions and distribution patterns: macule, papule, plaque, vesicle, bulla, pustule, and wheal. Common tested entities include eczematous dermatitis, psoriasis, drug eruptions, erythema multiforme, Stevens-Johnson syndrome and toxic epidermal necrolysis, cellulitis, stasis dermatitis, herpes zoster, scabies, urticaria, and melanoma. Melanoma uses ABCDE and requires excisional biopsy with narrow margins; shave biopsy can compromise staging. SJS and TEN are drug-induced emergencies with mucosal involvement and epidermal detachment, requiring drug withdrawal and burn-unit care.
Traps
- Calling bilateral lower-leg erythema cellulitis when it is stasis dermatitis.
- Missing mucosal involvement in a drug rash, which signals SJS or TEN.
- Using topical steroids on suspected infection without treating the infection.
- Shave-biopsying a pigmented lesion suspicious for melanoma.
- Forgetting that herpes zoster can involve the eye in V1 distribution.
- Treating urticaria with antibiotics instead of antihistamines and trigger removal.
Worked example
A patient started on allopurinol 10 days ago develops fever, painful mouth erosions, conjunctivitis, and dusky targetoid lesions with skin peeling. The diagnosis is SJS or TEN. Stop the drug immediately, provide supportive care, and involve dermatology and a burn unit. Rule of thumb: fever plus rash plus mucosal involvement equals a drug emergency until proven otherwise.