Inflammatory and Papulosquamous Dermatoses
What to master
This domain rewards pattern recognition plus mechanism. For eczema, know the atopic march, filaggrin loss-of-function, barrier dysfunction, and the acute (spongiotic, vesicular) versus chronic (lichenified) phases. For psoriasis, understand Th17/IL-23 axis, Koebner phenomenon, Auspitz sign, nail pitting, and the psoriatic arthritis screen. For lichen planus, learn the lichenoid interface dermatitis, Wickham striae, and mucosal/genital variants. For acne, distinguish comedonal, inflammatory, and nodulocystic disease and the role of Cutibacterium acnes, follicular hyperkeratinization, and androgen drive. For rosacea, know subtypes (erythematotelangiectatic, papulopustular, phymatous, ocular) and triggers.
Traps
- Psoriasis vs seborrheic dermatitis in scalp and flexures (inverse psoriasis lacks scale).
- Lichen planus vs lichenoid drug eruption - drug history and eosinophils help.
- Acneiform eruption from EGFR inhibitors or steroids mimics acne but lacks comedones.
- Rosacea is not infective; avoid antibiotic rationale based on culture.
Rule of thumb
Ask whether the primary lesion is a papule, plaque, vesicle, or comedo and whether scale is silvery (psoriasis), greasy (seborrheic), or absent (inverse). That single observation usually resolves the differential.
Worked example
A patient with well-demarcated erythematous plaques with silvery scale on elbows and knees plus nail pitting: the combination of extensor distribution, silvery scale, and nail changes points to psoriasis; the trap is choosing eczema because of itching.