Hidradenitis suppurativa (HS) is a chronic, relapsing inflammatory disease of hair follicles in intertriginous regions, characterized by painful nodules, abscesses, double-ended comedones, sinus tracts, and scarring. Pathogenesis involves follicular occlusion, dysregulated innate/adaptive immunity (IL-1, TNF, IL-17 axes), microbiome shifts, mechanical…

Sarcoidosis is a multisystem granulomatous disease characterized by noncaseating granulomas. Cutaneous sarcoidosis occurs in up to one-third of patients and may be the initial or sole manifestation. Skin lesions are polymorphic, including papules/plaques, lupus pernio, scar or tattoo infiltration, subcutaneous…

Cutaneous vasculitis encompasses a spectrum of disorders characterized by vessel wall inflammation resulting in purpura, nodules, ulcers, and livedoid changes. Small-vessel neutrophilic vasculitis—classically leukocytoclastic vasculitis (LCV), also termed cutaneous small-vessel vasculitis (CSVV)—presents with palpable purpura on dependent areas. Etiologies include…

Dermatomyositis (DM) is an idiopathic inflammatory myopathy characterized by pathognomonic cutaneous findings and variable muscle, lung, and systemic involvement. Cutaneous dermatomyositis (CDM) may occur with minimal or absent myositis (clinically amyopathic DM). Hallmark rashes include heliotrope erythema, Gottron papules/sign, photosensitive…

Cutaneous lupus erythematosus (CLE) encompasses a spectrum of lupus-specific skin diseases that may occur with or without systemic lupus erythematosus (SLE). Major subtypes include acute CLE (ACLE), subacute CLE (SCLE), and chronic CLE (CCLE; e.g., discoid lupus erythematosus, DLE). Pathogenesis…

Erythema multiforme (EM) is an acute, immune-mediated eruption characterized by target lesions, commonly triggered by herpes simplex virus (HSV). Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are severe, life-threatening mucocutaneous reactions—usually drug-induced—marked by epidermal necrosis and detachment. EM is…

Urticaria (hives) consists of transient, pruritic wheals with or without angioedema due to mast cell activation and histamine release. It is classified by duration into acute (<6 weeks) and chronic (≥6 weeks), and by triggers into spontaneous and inducible (physical)…

Pityriasis rosea (PR) is a self-limited papulosquamous eruption characterized by a herald patch followed by a secondary exanthem of oval, erythematous plaques with collarette scale along cleavage lines, producing a “Christmas-tree” pattern on the trunk. Human herpesviruses 6 and 7…

Lichen planus (LP) is a chronic, inflammatory, papulosquamous disorder affecting skin, mucous membranes, hair, and nails. It presents most commonly with pruritic, flat-topped, violaceous papules and plaques with Wickham striae. Mucosal involvement, particularly oral LP, can be chronic and erosive.…

Psoriasis is a chronic, immune-mediated disease characterized by well-demarcated, erythematous plaques with silvery scale, driven primarily by the IL-23/Th17 axis. It affects skin, nails, scalp, intertriginous areas, and may involve joints (psoriatic arthritis). Management depends on severity, distribution, comorbidities, and…