Morphology describes the general appearance of a rash and can be described in terms of primary and secondary lesions.1–3 Primary lesions are uncomplicated abnormalities which represent the initial pathologic change. Secondary changes reflect progression of disease such as excoriation, infection, or keratinization. The morphologic expression of a dermatologic condition is the basis on which all diagnoses are founded. In some cases, the appearance is distinctive enough to make a diagnosis at a glance (i.e., the grouped vesicular lesions of herpes simplex). In other cases, the appearance may be modified by scratching, secondary infection, or prior treatment. Most importantly, it is important to recognize that skin disease may evolve over time.
Macule: Circumscribed, flat discoloration of <1 cm in diameter; examples include ash-leaf spots, flat nevi, and freckles. Macules may constitute the whole or part of a rash or may represent the early phase.
Patch: Flat discoloration that is >1 cm in diameter and circumscribed; examples include vitiligo or tinea versicolor. Patches often have a very fine scale on the surface.
Papule: Circumscribed, superficial, solid elevated lesion <1 cm in diameter; examples include warts, elevated nevi, insect bites, and molluscum contagiosum.
Plaque: A lesion >1 cm in diameter elevated, flat top, superficial lesion; examples include psoriasis and pityriasis rosea.
Vesicle: Fluid-filled lesion <1 cm in diameter; examples include herpes simplex and varicella.
Bulla: Fluid-filled lesion >1 cm in diameter; examples include staphylococcal scalded skin syndrome and bullous impetigo.
Pustule: Pus-filled lesion; examples include acne and folliculitis.
Nodule: Lesion <1 cm in diameter with depth, may be subsurface, suprasurface, or at level of surface.
Tumor: Solid lesion >1 cm with depth, which may be subsurface, suprasurface, or at level of surface.
Petechiae: Pinpoint <1 cm flat, round red spots under the skin surface, because of deposits of blood and/or pigment; examples include drug eruption, Rocky Mountain spotted fever.
Purpura: Visible collection of blood/pigment >1 cm in diameter; examples include Henoch–Schonlein purpura and idiopathic thrombocytopenia purpura.
Wheal: Transient, edematous papule, or plaque with pale center and pink margin, peripheral erythema; examples include hives and insect bites.
Scale: Dry or greasy masses of keratin ranging from fine and delicate to coarse. Generally implies a pathologic process in the epidermis.
Crust: Dried exudate (serum, pus, or blood).
Excoriation: Punctate or linear abrasions generally caused by scratching.
Fissure: Linear crack or cleavage on skin, with sharply defined margins.
Ulcer: Depressed lesion because of epidermal or dermal tissue loss.
Scar: Permanent lesion resulting from the process of repair/replacement of connective tissue.
Lichenification: Area of increased epidermal thickness with accentuation of skin.