Medical Dermatology: Morphology, Timeline, and Systemic Clues
Start with morphology, then test the tempo
Medical dermatology questions reward a disciplined morphology-first approach. Describe the primary lesion (macule, papule, plaque, vesicle, pustule, nodule), its arrangement (linear, annular, grouped, dermatomal), and distribution (acral, flexural, photoexposed, mucosal). Then establish the tempo: acute versus chronic, and the relationship to drugs, infection, travel, or systemic symptoms.
Concepts that drive the differential
Know Koebnerization, the Auspitz sign, Nikolsky sign, and photodistribution. Recognize that psoriasis can mimic tinea, and that tinea incognito after topical steroids loses its annular scale. Stasis dermatitis is frequently mislabeled cellulitis. DRESS and SJS/TEN differ in timing, facial edema, eosinophilia, and mucosal burden. In blistering disease, pemphigus vulgaris shows suprabasal acantholysis with intercellular IgG, whereas bullous pemphigoid shows a subepidermal split with linear IgG/C3 at the basement membrane. Palpable purpura suggests vasculitis and warrants a biopsy for direct immunofluorescence when IgA vasculitis or urticarial vasculitis is considered.
Traps
Questions often hide a drug, an infection, or a systemic disease behind a common pattern. Mucosal involvement, fever, lymphadenopathy, atypical lymphocytosis, and organ dysfunction are red flags. Do not anchor on a previous diagnosis when the distribution has changed.
Worked rule
For any new rash in a patient taking a new medication, write the start date of every drug. The culprit is usually the agent started 1-8 weeks earlier, not one taken for years. Exception: a remembered sensitization can produce a reaction within hours to days.