Which stain is incorporated into the tooth surface and is difficult to remove?

The correct response identifies tetracycline as the stain that becomes incorporated into the tooth structure, making it particularly challenging to remove. Tetracycline stains result from the use of tetracycline antibiotics during the development of teeth, and they penetrate the dentin and enamel in a way that alters the color of the tooth itself. This incorporation means that even with professional cleaning or whitening treatments, the stain may remain because it is not just on the surface; it is part of the tooth's composition.

In contrast, intrinsic stains do affect the internal structure of the tooth, but common intrinsic stains are often due to factors other than medications and can sometimes be treated or lightened with specific dental procedures. Extrinsic stains, however, are only on the surface and can often be removed through standard dental hygiene practices, such as professional cleanings or whitening treatments. Green stains are typically surface stains created by specific factors like poor hygiene or the presence of certain bacteria, and they can be removed more easily than stains that have penetrated the tooth.