What causes a port wine stain?

A port wine stain is caused by an abnormal growth of blood vessels in the skin. This vascular malformation results in a permanent, flat, and often discolored mark on the skin that varies in color from light pink to deep burgundy, resembling the color of port wine. The condition arises due to the failure of the blood vessels to form properly during fetal development, leading to an overabundance of dilated blood vessels in the affected area.

The other options do not accurately describe the nature of a port wine stain. Viral and bacterial infections relate to the presence of pathogens affecting the skin or tissue rather than a structural anomaly of blood vessels. An allergic reaction involves an immune response to allergens that can result in hives or rashes but is not linked to the systemic vascular malformation characteristic of port wine stains. Understanding the correct cause helps in differentiating it from other skin conditions and informs appropriate management and treatment approaches.