Which of the following is a description of a nevus?

A nevus, commonly known as a mole or birthmark, is accurately described as a mark on the skin that can vary in shape and color. When considering option B, it emphasizes the irregular shapes that some types of nevi can have, distinguishing them from other skin marks which may be more uniformly shaped. Nevi can be present at birth or can develop over time and may vary significantly in appearance.

In context to the other options, while option A suggests a congenital skin disorder, not all nevi are disorders; many are simply benign growths. Option C refers to a skin condition causing dark patches, which does not specifically define a nevus as it could encompass various other conditions. Lastly, option D incorrectly categorizes a nevus as a type of skin cancer, whereas most nevi are benign and not cancerous unless changes occur that may necessitate further evaluation. Thus, option B best encapsulates the defining characteristics of a nevus.