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Differentiating between primary and secondary lesions can help develop the differential diagnosis list. Primary lesions develop spontaneously as a direct result of the underlying disease. The following are primary lesions:

 Papule: elevated domed skin lesion <1 cm

 Plaque: elevated skin lesion with a flat top >1 cm

 Nodule: elevated domed skin lesion >1 cm that is solid and typically extends to deeper skin layers

 Pustule: small, circumscribed elevation of skin containing pus

 Vesicle: circumscribed elevation of skin filled with fluid <1 cm

 Bulla: circumscribed elevation of skin filled with fluid >1 cm

 Macule: flat, distinct area of color change <1 cm

 Patch: flat area of color change >1 cm

 Wheal: elevation (often sharp) of the skin surface consisting of edema

 Cyst: membranous cavity or sac containing fluid or solid material

Secondary lesions can develop from primary lesions or evolve from patient/external factors (such as medications or scratching). They can be helpful, as they may indicate primary lesions that are no longer present, e.g. ulcer developing from vesicle or bulla. The following are secondary lesions:

 Epidermal collarette: circular ring of crust or scale (may be secondary to a pustule or new evidence suggests some may be primary lesions)

 Lichenification: hardened, thickened skin that is often a response to chronic inflammation or friction

 Erosion: epidermal defect that does not penetrate the basal laminar zone

 Ulcer: epidermal defect that exposes underlying dermis (deeper than erosion)

 Excoriation: erosion/ulcer caused by scratching (often linear), rubbing, or biting

 Fissure: cleavage into epidermis or dermis

 Callus: thickened, hyperkeratotic, alopecic plaque

 Scar: fibrous tissue that has replaced damaged dermis or subcutaneous tissue

Some lesions can be either primary or secondary. For instance, alopecia is a primary lesion in cases of hypothyroidism, but is secondary when caused by scratching. The following are primary or secondary lesions:

 Alopecia: loss of hair

 Comedo: hair follicle that is dilated and filled with sebaceous material and cornified cells

 Follicular casts: keratin and follicular material accumulation that adheres to hair shaft and extends beyond the surface of the follicular ostia

 Crust: dried exudate, serum, cells, blood, pus, and other materials adhered to skin

 Scale: loose fragments of cornified cells that accumulate on the skin surface

 Hyperpigmentation: darkening of skin color

 Hypopigmentation: lightening of skin color