A port wine stain:

A port wine stain is indeed classified as a birthmark. This type of birthmark is caused by an excess of capillaries near the surface of the skin, which creates a flat, discolored area that can range from light pink to dark red or purple. Port wine stains are typically present at birth and can vary in size and shape. They are not the result of injury, sun damage, or infections, but rather a vascular malformation that occurs during fetal development. Understanding that port wine stains are congenital helps in differentiation from other skin conditions and shapes the approach to treatment and care.