Extrinsic stains are primarily caused by what type of source?

Extrinsic stains are primarily caused by external sources, making this the correct choice. These types of stains are typically the result of substances that come into contact with the outer surface of the teeth, primarily enamel. Common external sources include foods and beverages like coffee, tea, red wine, and certain fruits, as well as tobacco products. The pigments and chemicals in these items can adhere to the tooth surface and lead to staining.

Understanding the nature of extrinsic stains emphasizes the importance of dental hygiene practices, as they can often be treated with regular brushing, professional cleanings, or whitening treatments. The other options relate more to internal factors or treatments that do not serve as the primary cause of extrinsic stains. Internal health conditions and dietary changes may affect the overall health and coloration of teeth but do not directly result in extrinsic staining. Fluoride application, on the other hand, is a preventive measure aimed at strengthening enamel and helping to avoid tooth decay, rather than a source of stains.