Medical Dermatology: Morphology, Patterns, and Systemic Clues
Morphology first
Medical dermatology questions reward a disciplined descriptive vocabulary: primary lesion, arrangement, distribution, and evolution. Learn to separate papulosquamous from spongiotic disease. Psoriasis shows sharply demarcated erythematous plaques with silvery scale on extensor surfaces, Auspitz sign, and nail pitting; eczema is ill-defined, flexural, and intensely pruritic. Trap: inverse psoriasis in the axilla or groin can mimic candidiasis; KOH and a search for satellite pustules settle it. Another classic trap is confusing nummular eczema with tinea corporis - annular lesions with central clearing demand a KOH examination before any steroid.
Blistering disease requires split-level logic: intraepidermal (pemphigus vulgaris, Nikolsky positive) versus subepidermal (bullous pemphigoid, tense, Nikolsky negative). Drug eruptions are high-yield: morbilliform exanthem (days 4–14), fixed drug eruption (recurring dusky plaque), DRESS (fever, eosinophilia, facial edema, internal organ involvement, 2–8 weeks), and SJS/TEN (mucosal erosions, epidermal detachment, positive Nikolsky).
Cutaneous signs of systemic disease must be recognized instantly: acanthosis nigricans (insulin resistance, malignancy), erythema nodosum (streptococcus, sarcoidosis, IBD, drugs), pyoderma gangrenosum (IBD, myeloma), and dermatomyositis (Gottron papules, shawl sign).
Rule of thumb: if a lesion is annular with central clearing, do a KOH test before diagnosing eczema or prescribing a steroid - tinea incognito is a favorite scenario. Worked example: a 34-year-old with an annular, scaly forearm plaque and mild itching; KOH shows branching hyphae, confirming tinea corporis and avoiding misdiagnosis as granuloma annulare or discoid eczema.