Inflammatory, Papulosquamous and Eczematous Disorders
Pattern recognition first
Most items in this domain test whether you can separate the major reaction patterns: spongiotic (eczema), psoriasiform (psoriasis, pityriasis rubra pilaris), lichenoid (lichen planus) and interface/vacuolar. Ask three questions: where is the rash (flexural, extensor, palmoplantar, scalp), what is the scale like (silvery and thick, fine and pityriasiform, collarette), and what do the nails and mucosae show?
The named disorders
Chronic plaque psoriasis: well-demarcated erythematosquamous plaques on extensor surfaces, Auspitz sign, Koebner phenomenon, nail pitting and onycholysis, and "oil-drop" spots. Guttate psoriasis follows streptococcal pharyngitis. Lichen planus: the six Ps - pruritic, purple, polygonal, planar papules and plaques - with Wickham striae and oral, genital and nail involvement. Atopic dermatitis: itch is essential; flexural in adults, face and extensor in infants, with xerosis and personal or family atopy. Seborrhoeic dermatitis favours nasolabial folds and scalp and flares with Parkinsonism and HIV. Pityriasis rosea gives a herald patch, then a "Christmas tree" truncal eruption with collarette scale.
Traps
- Scaly annular lesions are assumed to be tinea: always consider KOH microscopy, and remember tinea incognito after topical steroid.
- A palmoplantar scaly rash with little itch may be secondary syphilis - serology is the safe answer.
- Erythema multiforme (target lesions, acral, herpes-triggered) is a different disease from SJS/TEN (mucosal, drug-triggered, Nikolsky positive).
- Psoriasis is often itchy; itch does not exclude it.
Rule of thumb
If a rash is well demarcated, symmetrical and extensor, think psoriasis; if it is ill-defined, flexural and exquisitely itchy, think eczema.