Which of the following is an example of an endogenous stain?

Endogenous stains originate from within the body and are a result of internal factors rather than external substances. An excessive amount of fluoride during the development of teeth is classified as an endogenous stain because it occurs while the teeth are forming and can lead to dental fluorosis, which manifests as discoloration on the tooth surface.

In contrast, the other options refer to stains that are caused by substances originating outside the body. For example, stains from coffee, tobacco, and red wine are caused by pigments in those substances that adhere to the tooth surface. These are considered exogenous stains since they result from external factors and can typically be removed with professional dental cleaning or good oral hygiene practices. Thus, the choice of excessive fluoride as an example of an endogenous stain accurately captures the nature of how this type of stain occurs during the development of teeth.