A nevus is a small brown rash that almost everyone has. It is a developmental abnormality and is caused by localized aggregation of melanocytes due to accidental abnormalities during the movement of melanocytes from the neural crest to the epidermis again. Acquired pigmented nevi are associated with sun exposure, mechanical irritation of the skin, trauma, etc. The number of nevi increases with age, often increasing significantly during youthful development, and all are dark brown or black in color, with a few being colorless qualities without color. If you find that the pigmented mole suddenly becomes larger, unevenly pigmented, red, ulcerated and painful, you need to go to the dermatology department of the hospital as soon as possible, except for the possibility of malignant change.