The document discusses organizing the differential diagnosis of rashes based on the morphology of the primary lesion. The primary lesion must first be identified before categorizing the eruption's morphology. Common primary lesions include macules, papules, plaques, nodules, vesicles, and bullae. Differential diagnoses are then suggested based on the morphology and distribution of the primary lesion, such as papulosquamous eruptions including eczematous dermatitis, psoriasis, and tinea infections. Folliculopapular eruptions may be caused by acne, rosacea, or folliculitis. Dermal reaction patterns can result from conditions like urticaria, sarcoidosis,

1. 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 Lesions
Papulosquamous eruptions (papules and plaques)
 Eczematous dermatitis
o Atopic dermatitis
o Allergic contact dermatitis
o 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 Thrombocytopenia
o Medication related
o Benign pigmented purpura
o Bacteremia
o Disseminated intravascular coagulation
o Actinic/senile purpura
o Corticosteroid associated
o Amyloidosis
Blistering disorders (vesicles, pustules, and bullae)
 Autoimmune
o Bullous pemphigoid
o Pemphigus vulgaris
o Epidermolysis bullosa acquisita
 Congenital
o Epidermolysis bullosa
o Epidermolytic hyperkeratosis
 Infectious
o Varicella zoster
o Herpes simplex
o Impetigo
o Staphylococcal scalded skin
o Hypersensitivity syndromes
o Stevens-Johnson syndrome
o Toxic epidermal necrolysis