Dermatology: Morphology, Reaction Patterns, and Safe Next Steps
What this domain tests
The Dermatology domain tests whether you can convert a visual finding into precise words, build a short ranked differential, and choose the safe next step. Items reward morphology, reaction-pattern thinking, emergency recognition, and first-line management - including when to biopsy instead of treating empirically.
Describe before you diagnose
Name the primary lesion first: macule/patch (flat), papule/plaque/nodule (raised), vesicle/bulla (fluid), pustule (pus), wheal (transient edema), purpura (non-blanching). Secondary changes - scale, crust, erosion, ulcer, fissure, lichenification, atrophy, scarring - signal chronicity and behavior. Distribution (flexural, extensor, photo-exposed, dermatomal) and configuration (annular, linear, grouped, reticulated) narrow the differential more than color does.
Organize by reaction pattern
Papulosquamous: psoriasis (well demarcated, silvery, extensor), lichen planus (violaceous, polygonal, Wickham striae, flexural), pityriasis rosea (herald patch, Christmas-tree), tinea corporis (annular, leading scale), secondary syphilis (palms and soles). Spongiotic/eczematous: atopic, contact, stasis, dyshidrotic. Vesiculobullous: pemphigus vulgaris (intraepidermal, flaccid, mucosal, Nikolsky positive) versus bullous pemphigoid (subepidermal, tense, Nikolsky negative). Dermal and vascular: urticaria, leukocytoclastic vasculitis (palpable purpura), erythema multiforme.
Emergencies that decide the score
Stevens-Johnson/TEN (mucosal erosions, epidermal detachment, body-surface-area grading), DRESS (facial edema, eosinophilia, transaminitis, HHV-6 reactivation), AGEP (pustules, fever, neutrophilia), necrotizing fasciitis (pain out of proportion, crepitus, bullae), angioedema with airway compromise, purpura fulminans.
Worked example
A 34-year-old has intensely pruritic, violaceous, flat-topped polygonal papules on the flexural wrists plus white lacy oral streaks. Reasoning: the 6 Ps (pruritic, purple, polygonal, planar, papules, plaques) with Wickham striae and Koebnerization point to lichen planus; psoriasis is extensor with silvery scale, and secondary syphilis favors palms/soles with systemic signs. Biopsy shows a band-like lymphocytic infiltrate with sawtooth rete ridges; screen for hepatitis C.
Traps
Bilateral lower-leg "cellulitis" is usually stasis dermatitis; steroid-modified tinea incognito mimics eczema; Nikolsky sign is supportive, not diagnostic; mucosal involvement should never be attributed to a benign process; annular lesions are not automatically tinea.