Papulosquamous and Eczematous Disorders
Papulosquamous pattern
Psoriasis, lichen planus, pityriasis rosea, and seborrheic dermatitis share scaly papules/plaques but differ in distribution and histology. Psoriasis: well-demarcated erythematous plaques with silvery scale on extensor surfaces, Auspitz sign, Koebner phenomenon; histology shows regular acanthosis, parakeratosis, Munro microabscesses, and dilated capillary loops. Lichen planus: violaceous, pruritic, flat-topped papules with Wickham striae, oral involvement; histology shows band-like lymphocytic infiltrate, saw-tooth rete ridges, and Civatte bodies.
Eczematous pattern
Atopic dermatitis, contact dermatitis, seborrheic dermatitis, and stasis dermatitis share spongiosis histologically. Atopic dermatitis features flexural involvement, personal/family atopy, and filaggrin mutations; contact dermatitis is localized to exposure and confirmed by patch testing.
Traps
- Pityriasis rosea herald patch mistaken for tinea corporis; KOH negative and Christmas-tree distribution clarify.
- Seborrheic dermatitis vs psoriasis of scalp: psoriasis has thicker plaques extending beyond hairline.
- Lichen planus vs lichenoid drug eruption: drug eruption often has parakeratosis and eosinophils.
Rule of thumb
For any scaly plaque, ask three questions: distribution (extensor vs flexural vs dermatomal), mucosal involvement, and histology (spongiosis vs acanthosis vs interface).
Worked example
A 30-year-old with violaceous wrist papules and white oral striae: think lichen planus; biopsy shows band-like infiltrate with saw-tooth rete ridges.