Diagnostic Dermatology: History, Morphology, Dermatoscopy and Dermatopathology
Domain 2 tests the translation chain: lesion description, then differential, then bedside test or biopsy. Examiners reward precise morphology words and a defensible next step.
Morphology
- Primary lesions: macule/patch, papule/plaque/nodule, vesicle/bulla, pustule, wheal, purpura (non-blanching), telangiectasia.
- Configuration: annular, linear, grouped, zosteriform, reticulate, targetoid. Distribution: flexural, extensor, acral, photodistributed, dermatomal.
- History: onset, evolution, itch, drug timeline, occupation, travel and family history change the differential before the examination.
Dermatoscopy
- Two-step algorithm: melanocytic versus non-melanocytic. Melanocytic clues are pigment network, dots and globules, streaks, blue-white veil and regression.
- Vascular patterns: arborizing vessels in basal cell carcinoma, glomerular vessels in Bowen disease, dotted vessels in melanoma, comma vessels in dermal naevus.
- Acral lesions: parallel ridge pattern suggests melanoma; parallel furrow is benign. Nail: longitudinal band, Hutchinson sign, matrix examination.
Dermatopathology
- Pattern approach: interface, spongiotic, psoriasiform, lichenoid, vesiculobullous, granulomatous, panniculitis.
- Direct immunofluorescence: intercellular IgG in pemphigus, linear IgG/C3 at the basement membrane in pemphigoid, granular IgA in dermal papillae in dermatitis herpetiformis.
Worked example
A 48-year-old has a 7 mm irregular pigmented macule on the sole. Dermatoscopy shows a parallel ridge pattern and irregular dots. The correct step is excisional biopsy for Breslow measurement, not shave or cryotherapy. Reasoning: the ridge pattern indicates acral melanoma; thickness determines margins and sentinel node biopsy.
Traps
Do not shave a suspected melanoma. Do not interpret parallel furrow as malignancy. Do not call spongiotic dermatitis eczema without clinical correlation, because a drug reaction can look identical.