A nodule is a type of primary lesion defined as a limited, substantial, deep lesion that can be located deep in the dermis or subcutaneous tissue and can be caused by an inflammatory infiltrate such as erythema nodosum, or a deposit of metabolic products such as nodular xanthomas. The lesions are often round or oval in shape, may or may not be elevated on the skin surface, and need to be palpated in order to be seen, and are hard or infiltrative to the touch. The nodules may gradually resorb and fade, or they may break down and form ulcers, which usually form scar tissue after healing.