Final answer:
A port wine stain is a birthmark resulting from capillary malformations, causing a reddish to purplish discoloration on the skin.
Step-by-step explanation:
The correct definition of a port wine stain is a congenital malformation of capillaries in the skin, which causes a reddish to purplish discoloration that resembles the color of port wine.
Port wine stains are a type of birthmark and are also known by the medical term nevus flammeus. These lesions are usually present at birth and can occur anywhere on the body, but are most commonly found on the face, neck, scalp, arms, or legs. The size and shape can vary greatly, and the color may darken or lighten over time.
Although mostly a cosmetic concern, port wine stains can sometimes be associated with other syndromes or complications, thus requiring medical evaluation.