General and Medical Dermatology: Morphology, Reaction Patterns, and Systemic Clues
Core concepts
General dermatology tests whether you can describe a lesion precisely and then reason from that description. Master the primary lesion lexicon: macule, papule, plaque, nodule, vesicle, bulla, pustule, wheal, cyst. Then layer secondary changes (scale, crust, erosion, ulcer, lichenification, atrophy, scarring) and distribution (photodistributed, acral, intertriginous, dermatomal, mucosal).
Reaction patterns matter more than memorized disease lists. Learn to recognize spongiotic (eczematous, pruritic, vesicles), psoriasiform (well-demarcated plaques with silvery scale), interface (targetoid, photodistributed, mucosal involvement), granulomatous, and vasculopathic patterns. Each pattern narrows the differential before you ever reach a diagnosis.
Medical dermatology requires linking skin findings to systemic disease. Examples: erythema nodosum (tender pretibial nodules, associated with streptococcal infection, sarcoidosis, IBD, drugs), pyoderma gangrenosum (painful ulcer with violaceous undermined border, associated with IBD and hematologic disease), and acanthosis nigricans (velvety hyperpigmentation of flexures, insulin resistance).
Know the emergency red flags: mucosal involvement plus skin pain plus fever suggests a severe drug reaction; rapidly progressive purpura suggests vasculitis or infection; blistering with a positive Nikolsky sign suggests pemphigus or toxic epidermal necrolysis.
Traps
- Naming a diagnosis without describing morphology (questions often hinge on the description).
- Confusing cellulitis (ill-defined, warm, tender) with erysipelas (raised, sharply demarcated).
- Forgetting that photodistribution points to a drug or connective tissue disease.
- Overlooking mucosal involvement as a severity marker.
Rule of thumb
Describe before you diagnose: primary lesion + color + distribution + secondary change. A "tender pretibial nodule" is a pattern; erythema nodosum is the diagnosis that follows.