STEPTRACKERSKIN | PATHOLOGY + IMMUNOLOGY + PHARMACOLOGY | HIGH-YIELD REVIEW

Inflammatory Dermatoses

The common rashes — what they look like, where they appear, and the histologic word that names each one.

FOCUS ON
Psoriasis: silvery-scaled plaques on extensors, nail pitting; acanthosis, parakeratosis, Munro microabscesses; IL-23/Th17. Lichen planus: pruritic purple polygonal papules, Wickham striae, saw-tooth rete, band of lymphocytes; HCV. Atopic dermatitis: flexural, filaggrin, spongiosis. Allergic contact: type IV. Urticaria: type I wheals < 24 h.
DISORDER
MECHANISM
FEATURES · HISTOLOGY
CLUE · TREATMENT
Psoriasis
IL-23 → Th17 (IL-17) and TNF drive keratinocyte proliferation; HLA-Cw6; triggers: trauma (Koebner), strep, β-blockers, lithium
Well-demarcated salmon plaques with silvery scale on extensors, scalp; nail pitting; Auspitz sign · acanthosis, elongated rete, parakeratosis, ↓ granular layer, Munro microabscesses
Topical steroids + vitamin D analogs, UV, methotrexate, TNF/IL-17/IL-23 inhibitors
Lichen planus
T-cell attack on basal keratinocytes; associated with HCV
Pruritic, purple, polygonal, planar papules with Wickham striae (wrists, shins); lacy white oral lesions · saw-tooth rete ridges, band-like lymphocytes at the DE junction, Civatte bodies
Check HCV · topical steroids
Atopic dermatitis
Barrier defect (filaggrin) + Th2/IgE (type I); atopic triad with asthma, allergic rhinitis
Infants: face, extensors · older: flexures; itchy, lichenified · spongiosis
Emollients, topical steroids, dupilumab · eczema herpeticum risk
Allergic contact dermatitis
Type IV hypersensitivity to nickel, poison ivy (urushiol), neomycin, fragrances
Itchy vesicles in the shape of the exposure (watchband, linear streaks) · spongiosis
Patch testing · avoid allergen
Irritant contact dermatitis
Direct chemical injury (soaps, solvents), no sensitization
Dry, cracked, burning hands
Health workers, cleaners
Seborrheic dermatitis
Inflammatory response to Malassezia in sebum-rich areas
Greasy yellow scale on scalp (cradle cap), eyebrows, nasolabial folds, ears
Severe in Parkinson disease, HIV · ketoconazole
Urticaria
Mast cell degranulation (type I or direct) → dermal edema
Pruritic wheals, each lasting < 24 h; ± angioedema
Second-generation antihistamines
Pityriasis rosea
Probably HHV-6/7 reactivation
Single herald patch → oval scaly lesions along skin lines ("Christmas tree")
Self-limited (6–8 weeks)
Rosacea
Neurovascular dysregulation, Demodex mites
Central facial flushing, telangiectasias, papulopustules (no comedones); rhinophyma
Triggers: heat, alcohol, spicy food
Acne vulgaris
Androgens → sebum; follicular hyperkeratinization; Cutibacterium acnes; inflammation
Comedones, papules, pustules, nodules on face, chest, back
Retinoids · isotretinoin is teratogenic

Mechanism — psoriasis

TriggerTrauma, strep, drugs
→
Dendritic cells → IL-23
→
Th17 → IL-17, IL-22+ TNF
→
Keratinocyte hyperproliferationTurnover days, not weeks
→
Thick plaque, parakeratosis

Dilated tortuous capillaries in the dermal papillae bleed when scale is removed (Auspitz sign).

Histology words

TERM
MEANING
EXAMPLE
Hyperkeratosis
Thick stratum corneum
Psoriasis, calluses
Parakeratosis
Nuclei retained in stratum corneum
Psoriasis, actinic keratosis
Acanthosis
Thick stratum spinosum
Psoriasis, acanthosis nigricans
Spongiosis
Fluid between keratinocytes
Eczema, contact dermatitis
Acantholysis
Keratinocytes separate
Pemphigus vulgaris
Interface dermatitis
Lymphocytes attack basal layer
Lichen planus, lupus, EM

How it's tested

COMMON CONFUSIONPsoriasis favors extensors (elbows, knees) with silvery scale; atopic dermatitis in adults favors flexures (antecubital, popliteal) and is intensely itchy.
THE TRAPA new or worse psoriasis flare after starting a β-blocker or lithium — or after stopping systemic steroids (pustular flare) — is drug-related.
YOUR APPROACHMorphology (plaque, papule, vesicle, wheal) → distribution → itch → systemic association → histology term → treatment.

If you see… think…

Silvery-scaled plaques on elbows and knees with nail pitting → psoriasis
Neutrophils in the stratum corneum → Munro microabscesses (psoriasis)
Purple polygonal itchy papules on wrists + lacy white mouth lesions → lichen planus (check HCV)
Band of lymphocytes along a saw-tooth DE junction → lichen planus
Infant with itchy cheeks, asthma later → atopic dermatitis
Itchy vesicles under a new watchband → allergic contact dermatitis (nickel, type IV)
Greasy yellow scale on scalp and nasolabial folds → seborrheic dermatitis
Itchy wheals that vanish within hours → urticaria
Herald patch, then Christmas-tree rash → pityriasis rosea
Facial flushing, telangiectasias, bulbous nose → rosacea (rhinophyma)