Core Clinical Dermatology
Approach to the lesion
Core questions test morphologic description and pattern recognition. Be fluent in primary lesions (macule, papule, plaque, nodule, vesicle, bulla, pustule, wheal) and secondary changes (scale, crust, erosion, ulcer, lichenification, atrophy). Distribution matters: acral, photo-distributed, intertriginous, dermatomal, and follicular patterns each narrow the differential.
High-yield disease clusters
Papulosquamous disorders (psoriasis, lichen planus, pityriasis rosea, seborrheic dermatitis) hinge on scale color, border, and site. Eczematous disorders (atopic, contact, dyshidrotic) share spongiosis and pruritus. Urticaria is transient and wheal-based; angioedema implies deeper dermal involvement. Know the classic drug eruption patterns (morbilliform, fixed drug, DRESS, SJS/TEN) and their timelines.
Traps
Questions love look-alikes: psoriasis versus tinea (KOH), lichen planus versus lichenoid drug eruption, cellulitis versus stasis dermatitis. Another trap is management sequencing-e.g., choosing topical steroid potency by body site rather than by diagnosis alone. Beware "always biopsy" absolutes; many core questions test when biopsy is unnecessary.
Rule of thumb
For any erythematous scaly plaque, ask: does it favor extensor surfaces with silvery scale (psoriasis), flexures (seborrheic/inverse psoriasis), or annular with central clearing (tinea)? Site and scale color resolve most papulosquamous questions.
Worked example
A patient with well-demarcated plaques on elbows and knees with silvery scale and nail pitting: psoriasis. If the question adds a positive KOH with hyphae, the answer flips to tinea corporis-always check for the confirmatory test clue before committing.