Final answer:
Nevus simplex, also known as telangiectatic nevi, are flat, pink or red birthmarks caused by dilated blood vessels. They are harmless and may fade over time.
Step-by-step explanation:
Nevus simplex, also known as telangiectatic nevi, are flat, pink or red birthmarks that are present at birth or develop shortly after. They are caused by dilated blood vessels near the surface of the skin. Nevus simplex are typically harmless and do not require treatment. They can fade on their own over time or may persist throughout a person's life.