Medical Dermatology: Morphology to Safe Management
Medical Dermatology is the largest OED domain and tests whether you can move from morphology to a focused differential and a safe first-line plan. Questions favour common inflammatory disease, drug reactions, autoimmune blistering and connective-tissue disease, cutaneous infections, and urticaria, often with a systemic twist.
Core rules
- Describe before you diagnose: lesion type, colour, surface, border, arrangement, distribution, mucosal involvement and evolution. An annular plaque with central clearing suggests tinea, erythema annulare centrifugum or subacute cutaneous lupus; scale, KOH and photodistribution separate them.
- Psoriasis: well-demarcated erythematous plaques with silvery scale on extensors, scalp and nails; Auspitz sign is supportive, not diagnostic. Treat limited disease with topical corticosteroid plus vitamin D analogue; screen for metabolic syndrome, psoriatic arthritis and depression.
- Eczema: ill-defined, lichenified, excoriated lesions; check for secondary infection and allergens. Acute vesicular hand eczema needs potassium hydroxide and patch testing.
- Drug eruptions: morbilliform is common; red flags are mucosal erosion, blistering, fever, facial oedema, lymphadenopathy and eosinophilia. Stop the culprit and consider SJS/TEN, DRESS or AGEP.
- Autoimmune blistering: bullous pemphigoid is tense subepidermal with eosinophils; pemphigus vulgaris is flaccid with acantholysis. Direct immunofluorescence is decisive: linear IgG/C3 at the basement membrane vs intercellular IgG.
Worked example
A 34-year-old woman has 3 weeks of annular, photodistributed plaques on the upper chest with fine scale and no mucosal lesions; KOH is negative. Subacute cutaneous lupus is likely because of photodistribution, annular morphology and absence of hyphae. Check ANA and anti-Ro/SSA, review drugs, and use photoprotection plus topical steroid; systemic therapy if extensive.
Traps
Do not label every annular plaque as tinea without KOH. Do not miss mucosal erosions in a drug rash. Do not treat suspected cellulitis without considering stasis dermatitis or DVT. Never start systemic steroids before excluding infection when blistering is present.