Common Skin Lesion Morphology

Mechanism-based causes

• Infectious: viral cytopathic effect (ballooning degeneration in HSV/VZV) produces grouped vesicles; bacterial folliculitis and impetigo produce follicular pustules and honey-colored crust; dermatophyte hyphae in the stratum corneum produce annular scaly patches with central clearing
• Immune-mediated: IgE-driven mast cell degranulation causes transient wheals; type IV delayed hypersensitivity causes linear vesicular contact dermatitis; autoantibodies determine blister depth — anti-desmoglein (pemphigus vulgaris) gives fragile intraepidermal erosions, anti-hemidesmosome/BP180 (bullous pemphigoid) gives tense subepidermal bullae
• Neoplastic/proliferative: keratinocyte or melanocyte clonal expansion produces persistent papules, nodules, and tumors; UV-induced p53 and PTCH mutations underlie actinic keratosis, SCC, and BCC
• Vascular/hemostatic: red cell extravasation from thrombocytopenia or vessel-wall inflammation produces non-blanching petechiae and purpura; palpable purpura implies leukocytoclastic vasculitis rather than simple platelet failure
• Physical/self-induced: scratching produces excoriation, chronic rubbing produces lichenification, and aberrant collagen deposition after injury produces keloid
• Drug-induced: morbilliform exanthem, fixed drug eruption (recurrent annular plaque at the same site), and full-thickness keratinocyte apoptosis in SJS/TEN

Modifiable risk factors

• Cumulative UV and tanning-bed exposure: the dominant modifiable driver of keratinocyte carcinoma and melanoma; sun-protective behavior is the basis of USPSTF counseling recommendations for fair-skinned patients
• Immunosuppression: transplant immunosuppressants, chronic systemic corticosteroids, and HIV markedly increase SCC, warts, and disseminated zoster
• Barrier disruption and mechanical factors: scratching, occlusion, heat/humidity, obesity with intertrigo, and misapplied high-potency topical steroids
• Culprit drugs: aromatic anticonvulsants, sulfonamides, allopurinol, and NSAIDs for severe cutaneous adverse reactions
• Poor glycemic control: predisposes to candidal pustules and neuropathic ulceration

Non-modifiable risk factors

• Fitzpatrick phototype I–II, red hair, numerous or dysplastic nevi, and family history of melanoma
• Advanced age (bullous pemphigoid, actinic damage) and atopic genotype (filaggrin loss-of-function)
• HLA associations: HLA-B*15:02 with carbamazepine SJS/TEN in patients of Southeast Asian ancestry (FDA-recommended pre-treatment screening) and HLA-B*57:01 with abacavir hypersensitivity

• Primary lesions arise from specific inflammatory, neoplastic, or infectious processes — inflammation produces erythema and edema (macules/papules), vascular proliferation creates nodules and tumors, and viral/bacterial infection triggers characteristic morphologic patterns
• Secondary lesions result from temporal evolution and patient modification — scratching causes excoriation, secondary bacterial infection leads to crusting, and chronicity causes lichenification and scale formation
• Size and depth classification reflects anatomic involvement — macules and patches are flat and non-palpable (epidermal), papules and plaques are palpable surface elevations (dermal/epidermal), while nodules and tumors represent deeper dermal/subcutaneous involvement with greater clinical significance
• Color variations indicate different pathophysiologic mechanisms — erythema reflects vasodilation/inflammation, hyperpigmentation indicates melanin deposition or post-inflammatory change, and violaceous color suggests vascular involvement or tissue infiltration (as in lymphoma)
• Blanching versus non-blanching lesions distinguish vascular from structural pathology — blanching erythema indicates vasodilation and resolves with pressure, while non-blanching lesions (petechiae, purpura) represent true structural changes (RBC extravasation, melanin deposition) and do not blanch

• Macule (≤1 cm) vs. Patch (>1 cm) — flat, non-palpable lesions with altered color; macules are the smallest skin lesion and include freckles, vitiligo patches, and early viral exanthems; patches represent coalescence of macules or primary large lesions such as port-wine stains
• Papule (≤1 cm) vs. Plaque (>1 cm) — palpable surface elevations representing dermal inflammation; papules include acne, warts, and lichen planus; plaques are characteristic of psoriasis, mycosis fungoides, and lichenoid drug reactions
• Vesicle (≤1 cm) vs. Bulla (>1 cm) — fluid-filled intraepidermal cavities; vesicles typify varicella, herpes simplex, and dyshidrotic eczema; bullae are seen in bullous pemphigoid, pemphigus vulgaris, and epidermolysis bullosa
• Pustule — pus-containing lesion smaller than 1 cm; classic presentation in acne, impetigo, and folliculitis; important to distinguish from vesicles (transparent, sterile fluid) versus pustules (opaque, infected)
• Nodule (>1 cm, dermal/subcutaneous) vs. Tumor (>2 cm) — deeper, firm lesions; nodules include lipomas, erythema nodosum, and cystic acne; tumors are concerning for malignancy (melanoma, basal cell carcinoma)
• Wheal (urtica) — transient, edematous, blanching papule or plaque appearing within minutes and resolving within 24 hours; pathognomonic for urticaria; pruritic and often seen in IgE-mediated reactions
• Telangiectasia — permanently dilated superficial blood vessels appearing as red or violaceous linear or punctate lesions; seen in rosacea, hereditary hemorrhagic telangiectasia, and scleroderma

• Scale — dried serum, keratin, or epidermis; fine scales suggest fungal infection or psoriasis, while thick plaques with silvery scale are classic for plaque psoriasis
• Crust — dried serum or pus; yellow crusts suggest impetigo or seeping eczema, while hemorrhagic crusts may indicate trauma or vasculitis
• Erosion — loss of epidermis only (heals without scarring); seen after vesicle/bulla rupture in herpes simplex, varicella, or pemphigus
• Ulcer — loss of epidermis and dermis (heals with scarring); diabetic ulcers, venous insufficiency ulcers, and pressure ulcers are major clinical entities; deep ulcers suggest malignancy or severe vascular disease
• Excoriation — linear erosion from scratching; indicates severe pruritus and is seen in atopic dermatitis, scabies, and pruritus
• Lichenification — thickened, leathery skin with accentuated normal skin lines from chronic rubbing; characteristic of chronic eczema and lichen planus
• Atrophy — thinned skin appearing shiny or translucent; occurs from chronic corticosteroid use, aging, or scar formation
• Scar — permanent fibrotic replacement of dermis; hypertrophic scars remain within wound boundaries while keloids extend beyond original injury (pathologic healing response)

LESION CONFIGURATION AND ARRANGEMENT

• Grouped/clustered — characteristic of herpes simplex and herpes zoster ("crops" of vesicles)
• Linear — suggests contact dermatitis (poison ivy), trauma, or dermatitis artefacta
• Annular (ring-shaped) — seen in tinea corporis, erythema multiforme, and fixed drug eruption
• Target/iris lesions — three-zone appearance (central erythema/vesicle, pale middle zone, outer erythematous ring) pathognomonic for erythema multiforme
• Koebner phenomenon — new lesions appearing at sites of trauma; classic in psoriasis, lichen planus, and vitiligo
• Dermatomal distribution — unilateral, follows sensory dermatome; diagnostic of herpes zoster

• Clinical morphologic examination — describe lesion using standardized terminology (primary vs. secondary lesion type, size in centimeters, color, configuration, number, distribution/location); organize differential diagnosis based on key features (e.g., vesicular eruption narrows to herpes simplex, varicella, or dyshidrotic eczema); documentation with photography is high-yield for tracking evolution and medicolegal purposes
• Pattern recognition and associated symptoms — correlate morphology with timing (acute versus chronic), associated pruritus/pain, seasonal variation, and systemic symptoms; acute vesicular eruption with dermatomal pain strongly suggests herpes zoster; chronic plaques with silvery scale suggest psoriasis; acute papules on extensor surfaces suggest lichen planus
• Dermoscopy — handheld magnification revealing subsurface features; ABCDE criteria (Asymmetry, Border irregularity, Color variation, Diameter >6 mm, Evolving) identify concerning melanomas; dermoscopy distinguishes benign nevi (reticular pattern, regular globules) from melanoma (irregular dots/globules, blue-white veil); useful for identifying tinea (comma hairs, corkscrew hairs) versus seborrheic keratosis
• Biopsy indications and techniques — indicated for any lesion suspicious for malignancy, atypical presentation, or diagnostic uncertainty; punch biopsy is standard for inflammatory/infectious lesions (full-thickness specimen for pathology); shave biopsy for suspected melanoma is CONTRAINDICATED (cannot assess depth/Breslow thickness); excisional biopsy preferred for melanoma; allows histopathologic confirmation of diagnosis and assessment of severity (e.g., degree of dysplasia in actinic keratosis)
• Fungal studies — KOH preparation identifies pseudohyphae and yeast; fungal culture on appropriate media (Sabouraud dextrose agar) confirms species; Wood's lamp (365 nm UV light) screens for tinea versicolor (yellow

Immediate stabilization (morphology that signals emergency)

• Wheals with airway compromise, hypotension, or GI symptoms: treat as anaphylaxis — epinephrine 0.3 mg IM into the anterolateral thigh, repeated as needed, per the AAAAI/ACAAI Joint Task Force anaphylaxis parameter; antihistamines and steroids are adjuncts only and never replace epinephrine
• Dusky macules, targetoid lesions, positive Nikolsky sign, or mucosal erosions: stop the suspected drug immediately and transfer to a burn or ICU setting for fluid, wound, and ocular care
• Bullae or ecchymosis over a tender extremity with pain out of proportion: emergent surgical exploration for necrotizing soft tissue infection with broad-spectrum antibiotics, per IDSA skin and soft tissue infection guidelines

First-line therapy by primary lesion

• Inflammatory papules/plaques: topical corticosteroids (e.g., triamcinolone), potency matched to site and severity, per AAD atopic dermatitis and psoriasis guidelines; topical calcineurin inhibitors (tacrolimus) for face and intertriginous skin to avoid atrophy
• Wheals: second-generation H1 antihistamine (e.g., cetirizine), up-titrated above standard dose before adding other agents, per the Joint Task Force urticaria practice parameter
• Vesicles of HSV/VZV: nucleoside analog antivirals (e.g., valacyclovir), most effective when started early
• Pustules/crusted lesions: topical mupirocin for limited impetigo; oral antistaphylococcal agents (cephalexin, or doxycycline/TMP-SMX when MRSA is likely) for extensive disease; incision and drainage is the definitive treatment for a fluctuant abscess (IDSA)
• Annular scaly patches: topical azole or allylamine (terbinafine); oral terbinafine or griseofulvin for tinea capitis and onychomycosis

Escalation

• Phototherapy (narrowband UVB), then systemic immunomodulators — methotrexate, or biologics targeting TNF, IL-17, or IL-23 for psoriasis (AAD/National Psoriasis Foundation), dupilumab for refractory atopic dermatitis, omalizumab for antihistamine-refractory chronic urticaria

Definitive/surgical

• Neoplastic nodules and tumors: excision; melanoma requires wide local excision with margins determined by Breslow depth, with sentinel node biopsy considered per NCCN; Mohs micrographic surgery for high-risk or facial keratinocyte carcinoma per AAD appropriate use criteria

Contraindicated

• Systemic corticosteroids for plaque psoriasis (rebound pustular flare on withdrawal), topical steroids on undiagnosed scaly annular lesions (tinea incognito), prolonged high-potency steroids on face or under occlusion, and shave biopsy of suspected melanoma

Complications of the underlying lesion

• Secondary bacterial infection (impetiginization): barrier loss from excoriation or ruptured vesicles allows S. aureus or S. pyogenes entry; signaled by golden-yellow crust, expanding warmth, and purulence, and can progress to cellulitis, bacteremia, or post-streptococcal glomerulonephritis
• Necrotizing soft tissue infection: fascial-plane spread with microvascular thrombosis; heralded by pain out of proportion, hemorrhagic bullae, crepitus, and anesthesia over the lesion — a surgical emergency
• Non-blanching petechiae/purpura with fever: extravasation from endothelial injury and DIC; suggests meningococcemia or Rocky Mountain spotted fever — an emergency requiring empiric antibiotics before confirmatory testing
• Progression of erosion to ulcer: full-thickness dermal loss heals with scar; chronic non-healing ulcers with rolled or everted borders raise concern for SCC arising in the wound (Marjolin ulcer)
• Post-inflammatory hyperpigmentation or hypopigmentation: melanin transfer to dermal macrophages after inflammation; especially prominent and prolonged in darker Fitzpatrick phototypes
• Lichenification and keloid formation: chronic rubbing thickens epidermis; aberrant type III/I collagen deposition extends fibrosis beyond the original wound margin (versus hypertrophic scar, which stays within it)
• Missed melanoma: shave sampling transects the tumor base and prevents accurate Breslow measurement, understaging the patient and misdirecting excision margins

Complications of treatment

• Topical corticosteroid effects: fibroblast and collagen suppression cause atrophy, striae, telangiectasia, and purpura; overuse on the face causes perioral dermatitis and steroid rosacea; potent agents under occlusion or over large surface areas can cause HPA axis suppression, especially in children with high body-surface-to-weight ratio
• Tinea incognito: steroid-induced local immunosuppression blunts the annular border while the dermatophyte spreads, producing an atypical, poorly demarcated plaque
• Systemic steroid withdrawal in psoriasis: rebound generalized pustular psoriasis with fever and leukocytosis — an emergency with risk of hypovolemia and sepsis
• Biologic and immunosuppressant therapy: latent tuberculosis reactivation with TNF inhibitors (screen before initiation) and hepatotoxicity or cytopenias with methotrexate
• Antimicrobial adverse effects: TMP-SMX and other sulfonamides are among the drugs most associated with SJS/TEN — new mucosal erosions or a positive Nikolsky sign during therapy demands immediate drug cessation

• The 1 cm rule governs almost every morphology question: macule→patch, papule→plaque, vesicle→bulla all cross at 1 cm; nodules are >1 cm with dermal or subcutaneous depth. If the stem gives a size, it is testing this boundary
• A lesion that comes and goes within 24 hours is a wheal, not a papule: individual urticarial lesions lasting >24 hours or leaving bruising point to urticarial vasculitis and warrant biopsy rather than more antihistamine
• Blanching versus non-blanching is the single best bedside discriminator: blanching = vasodilation (viral exanthem, drug eruption); non-blanching = extravasated red cells (petechiae, purpura). Non-blanching lesions plus fever = immediate empiric antibiotics and blood cultures, not observation
• Nikolsky sign localizes the split: sloughing with lateral pressure indicates intraepidermal or dermoepidermal separation (pemphigus vulgaris, SJS/TEN, staphylococcal scalded skin); tense bullae in an elderly patient that resist Nikolsky suggest subepidermal bullous pemphigoid
• The single best next step for a pigmented lesion with ABCDE features is full-thickness biopsy — narrow excisional or deep punch. Shave biopsy is the classic wrong answer because it destroys Breslow depth, the strongest prognostic factor (NCCN)
• Configuration is a free diagnosis: three-zone target lesions = erythema multiforme (HSV-associated); dermatomal grouped vesicles = zoster; linear vesicles = plant contact dermatitis; annular with central clearing and scale = tinea; new lesions in a scratch line = Koebner phenomenon (psoriasis, lichen planus, vitiligo)
• Distractor to avoid — vesicle versus pustule: clear fluid means viral or spongiotic; opaque purulent fluid means neutrophilic (bacterial folliculitis, but also sterile pustules in pustular psoriasis and acute generalized exanthematous pustulosis). Pustule does not automatically mean infection
• Scar boundaries are tested verbatim: hypertrophic scars stay within the wound; keloids extend beyond it and recur after simple excision