A lesion is any abnormal, or pathologically altered, tissue on or within an organism. Types of primary lesions include vesicles, pustules, wheals (welts) and tumors whilst secondary lesions include crusts, pigmentations, scales, scars and ulcers. They may be caused by any process that damages tissues from cancer to trauma and are classified by their location such as brain lesion or skin lesion.

Because the definition of a lesion is very broad, it is often of very little use in a differential diagnosis.

Lesion at Wikipedia

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