Inflammatory and Papulosquamous Dermatoses
Domain 3 covers the commonest clinic encounters and tests pattern recognition plus treatment sequencing: eczema versus psoriasis versus lichenoid dermatitis, and when a drug is the culprit.
Psoriasis
- Well-demarcated erythematous plaques with silvery scale on extensor surfaces, scalp, umbilicus and gluteal cleft; Auspitz sign and Koebner phenomenon.
- Nails: pitting, onycholysis, oil spots. Psoriatic arthritis affects up to a third, so ask about joints.
- Driven by IL-23/Th17. Severity guides therapy from topical vitamin D analogue to NBUVB, methotrexate or biologics.
Lichenoid and interface dermatoses
- Lichen planus: pruritic, purple, polygonal, planar papules with Wickham striae; oral and genital involvement; check hepatitis C.
- Interface dermatitis on biopsy shows basal vacuolation and a lichenoid infiltrate. Lichenoid drug eruption is often widespread and photo-distributed and resolves on withdrawal.
Eczema group
Atopic dermatitis is flexural and pruritic, filaggrin-associated, with Staphylococcus aureus colonization. Contact dermatitis is geometric and confined to the exposure. Seborrheic dermatitis favours scalp, eyebrows and nasolabial folds.
Worked example
A 34-year-old has violaceous flat-topped papules on the flexor wrists and white lacy lines on the buccal mucosa. This is lichen planus. Reasoning: colour, configuration, Wickham striae and mucosal involvement; order hepatitis C serology and start topical corticosteroid.
Traps
Do not diagnose psoriasis when scale is greasy and sites are seborrheic; do KOH to exclude tinea. Do not miss lichenoid drug eruption after a new medication. Do not use potent steroids long term on the face.