General Dermatology: Morphology, Diagnosis, and Core Therapeutics
Start with morphology, not the diagnosis
Most general dermatology questions are morphology questions in disguise. Build a habit: identify the primary lesion (macule, papule, plaque, nodule, vesicle, bulla, pustule, wheal), then describe distribution (acral, flexural, extensor, dermatomal, sun-exposed), arrangement (linear, annular, grouped, reticulated), and evolution (acute vs chronic, scarring vs non-scarring). Terms like "herpetiform" describe grouping, not herpesvirus; "targetoid" describes concentric rings, not necessarily erythema multiforme.
High-yield concept clusters
Know the immunobullous diseases by their split level and immunofluorescence pattern: pemphigus vulgaris is intraepidermal with intercellular IgG ("chicken wire"); bullous pemphigoid is subepidermal with linear IgG/C3 at the basement membrane. Know that psoriasis is a neutrophilic, Th17-driven plaque disorder with Auspitz sign and Koebner phenomenon, and that its key mimics are tinea, nummular eczema, and lichen simplex. Know contact dermatitis as type IV delayed hypersensitivity, and patch testing as the diagnostic standard. Know that urticaria lesions resolve within 24 hours per site; fixed lesions suggest urticarial vasculitis.
Therapeutics and traps
Steroid potency by vehicle matters: ointments are more potent than creams, and thin skin (face, intertriginous) absorbs more. Chronic topical steroid use causes atrophy, striae, and telangiectasia, not just "rebound." Isotretinoin requires pregnancy prevention and monitoring of lipids and transaminases. Methotrexate requires folate supplementation and monitoring; it is teratogenic.
Worked example: A patient has annular scaly plaques with central clearing on the trunk. Central clearing plus an active scaly border points to tinea corporis, not psoriasis (which is uniformly scaly). A KOH prep confirms hyphae. The trap answer is a topical steroid, which would produce tinea incognito.
Rule of thumb: If a lesion has central clearing, think infection or granuloma annulare before eczema.