A non-vital tooth that has darkened is an example of what type of stain?

A non-vital tooth that has darkened is indicative of an intrinsic stain, which occurs from changes within the tooth itself. This type of staining can result from various factors, such as pulpal necrosis, internal bleeding, or the effects of certain medications that affect the tooth's natural color from within.

The term 'endogenous' refers to stains that originate from within the body—such as those caused by systemic factors—pointing to the intrinsic nature of the discoloration. Intrinsic stains are not removable by professional cleaning, which distinguishes them from extrinsic stains that are exterior and can be polished off. The combination of these terms highlights that the darkening of a non-vital tooth arises from internal processes rather than external influences, thus affirming that the correct classification for the stain described is intrinsic and more particularly endogenous.