A type of skin blemish of which there are 2 main groups: pigmented naevi are caused by abnormality or overactivity of melanocytes (skin cells that produce the pigment melanin); vascular naevi are caused by an abnormal collection of blood vessels. The most common types of pigmented naevi are freckles, lentigos, and café au lait spots: flat brown areas that may occur where the skin is exposed to the sun. Another common type is a mole, sometimes called a melanocytic naevus. In rare cases, moles become cancerous (see melanoma, malignant). Juvenile melanomas (see melanoma, juvenile) are red-brown naevi that occur in childhood. Blue naevi are common in young girls. Most black and Asian infants are born with blue-black spots on their lower backs (see Mongolian blue spot). Port-wine stains and strawberry marks (see haemangioma and spider naevi) are examples of vascular naevi. Most naevi are harmless. However, if a naevus suddenly appears, grows, bleeds, or changes colour, medical advice should be sought immediately to exclude the possibility of skin cancer.


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