Stern, Scott (2009-09-16). Symptom to Diagnosis: An Evidence Based Guide, Second Edition (LANGE Clinical Medicine) (Kindle Locations 17856-17895). McGraw-Hill. Kindle Edition.

The most useful way of organizing the differential diagnosis of a rash is to base it on the morphology of the lesion. To correctly categorize a lesion’s morphology, the physician must first identify the primary lesion, the typical element of the eruption.

Once the primary lesion is identified, the eruption can be categorized based on morphology and then the specific diagnosis identified. This process can be difficult. The primary lesion is often affected by secondary changes such as excoriation, erosion, crusting, and even coalescence.

The differential diagnosis of one lesion can also be extensive. After determining the morphology of the primary lesion, the next step in making the diagnosis is often to observe the distribution of lesion. Some eruptions will have characteristic distributions. What follows are some important definitions, followed by a differential diagnosis of some of the most common primary lesions.

Macule: lesion without elevation or depression, < 1 cm Patch: lesion without elevation or depression, > 1 cm Papule: any solid, elevated “bump” < 1 cm Plaque: raised plateau-like lesion of variable size, no depth, often a confluence of papules Nodule: solid lesion with palpable elevation, 1–5 cm Tumor: solid growth, > 5 cm Cyst: encapsulated lesion, filled with soft material Vesicle: elevated, fluid-filled blister, < 1 cm Bulla: elevated, fluid-filled blister, > 1 cm Pustule: elevated, pus-filled blister, any size Wheal: inflamed papule or plaque formed by transient and superficial local edema Comedone: a plug of keratinous material and skin oils retained in a follicle; open is black, closed is white

Papulosquamous eruptions present with papules and plaques associated with superficial scaling. Folliculopapular eruptions begin as papules arising in a perifollicular distribution. Dermal reaction patterns result from infiltrative and inflammatory processes involving the dermal and subcutaneous tissues. Petechia and purpura occur when there is leakage of blood products into surrounding tissues from inflamed or damaged blood vessels. Blistering disorders present with vesicles and bullae.

Differential Diagnosis of Most Common LesionsPapulosquamous eruptions (papules and plaques)

Eczematous dermatitiso Atopic dermatitiso Allergic contact dermatitiso Irritant contact dermatitis

Pityriasis rosea Tinea infections Psoriasis Seborrheic dermatitis

Folliculopapular eruptions (perifollicular papules) Acne vulgaris Rosacea Folliculitis Perioral dermatitis

Dermal reaction patterns Urticaria Sarcoidosis Granuloma annulare Erythema nodosum

Purpura and petechiae Palpable purpura

o Leukocytoclastic vasculitis Henoch-Schönlein purpura Allergic vasculitis

o Infectious Bacteremia Rocky Mountain spotted fever

Nonpalpable purpura o Thrombocytopeniao Medication related o Benign pigmented purpurao Bacteremia o Disseminated intravascular coagulationo Actinic/senile purpura o Corticosteroid associated o Amyloidosis

Blistering disorders (vesicles, pustules, and bullae) Autoimmune

o Bullous pemphigoido Pemphigus vulgariso Epidermolysis bullosa acquisita

Congenitalo Epidermolysis bullosa o Epidermolytic hyperkeratosis

Infectiouso Varicella zostero Herpes simplexo Impetigoo Staphylococcal scalded skino Hypersensitivity syndromeso Stevens-Johnson syndromeo Toxic epidermal necrolysis