Clinical Assessment and Diagnostic Reasoning in Dermatology
The morphology-first method
Every clinical question rewards a disciplined sequence: identify the primary lesion (macule, papule, plaque, nodule, vesicle, bulla, pustule, wheal), describe its secondary change (scale, crust, erosion, ulcer, lichenification, atrophy, scarring), then commit to a distribution and pattern (flexural, extensor, dermatomal, photodistributed, acral, follicular, Blaschkoid). Only then generate a differential and choose the test that discriminates.
Concepts to master
- Precise terminology: annular, reticular, serpiginous, umbilicated, dermatomal.
- Configurations that carry diagnostic weight: linear (contact, Koebner, incontinentia pigmenti), grouped (herpesviruses), targetoid (erythema multiforme).
- The Nikolsky sign and its variants, Darier sign, Auspitz sign, Koebner phenomenon.
- Test selection logic: when a KOH, scabies preparation, Tzanck, bacterial swab, punch biopsy, or direct immunofluorescence changes management, and when it merely confirms a clinical diagnosis.
Traps
Questions love to describe a lesion by symptom rather than morphology, or to offer a plausible diagnosis that ignores a single discordant feature (e.g., "scaly annular plaque with central clearing" steers to tinea, but an overhanging scale edge and absence of follicular involvement matter). Another classic trap is confusing distribution with diagnosis: seborrheic dermatitis and tinea versicolor both favor the trunk, but only one is KOH-positive with spaghetti-and-meatballs hyphae.
Rule of thumb
If two diagnoses remain after morphology and distribution, choose the test whose negative result changes management. A negative KOH in a scaly annular plaque justifies biopsy; a positive one ends the workup.