Clinical Dermatology: Inflammatory and Papulosquamous Disorders
Scope
The exam tests pattern recognition plus mechanism: psoriasis, lichen planus, atopic dermatitis, pityriasis rosea, pityriasis rubra pilaris, granulomatous and neutrophilic dermatoses. Expect morphology, dermoscopy, first-line therapy, and recognition of dangerous variants.
Core rules
- Psoriasis: well-demarcated erythematous plaques with silvery scale; Auspitz sign and Koebner phenomenon; IL-23/Th17 pathway; guttate flares follow streptococcal pharyngitis; treat with topicals, NB-UVB, methotrexate, or biologics (TNF, IL-17, IL-23p19).
- Lichen planus: pruritic violaceous, polygonal, flat-topped papules; Wickham striae; oral and genital involvement; histology shows band-like lymphocytic infiltrate, wedge-shaped hypergranulosis, saw-tooth rete ridges, Civatte bodies.
- Atopic dermatitis: xerosis, flexural involvement, filaggrin deficiency, S. aureus colonization; Th2/IL-4/IL-13 drive disease; dupilumab blocks IL-4Rα.
- Pityriasis rosea: herald patch, Christmas-tree distribution, HHV-6/7 association; self-limited.
- Pityriasis rubra pilaris: orange-red plaques with islands of sparing, follicular keratoses, palmoplantar keratoderma; CARD14 mutations in familial cases.
Worked example
A 28-year-old has annular scaly plaques on the elbows. Dermoscopy shows dotted vessels and white scale; a KOH test is negative. Reasoning: dotted vessels plus regular white scale favor psoriasis over tinea or nummular eczema. If Wickham striae and violaceous color were present, lichen planus would be favored.
Traps
Systemic corticosteroids can precipitate pustular psoriasis after withdrawal in guttate disease. Methotrexate is weekly, not daily. Nail pitting alone does not confirm psoriasis without skin or family evidence.