Which of the following is an intrinsic stain?

An intrinsic stain refers to discoloration that occurs within the tooth structure itself, rather than on the surface. Dental fluorosis is a perfect example of an intrinsic stain because it results from excessive fluoride exposure during the developmental stages of the teeth. This condition manifests as changes in the mineralization of the enamel, resulting in varying degrees of discoloration that are integrated into the tooth matrix, making them difficult to remove through regular cleaning methods.

In contrast, the other options represent extrinsic stains that occur on the tooth's surface due to external factors. For instance, tea and nicotine stains are primarily caused by substances that are directly applied to or in contact with the teeth, while tartar (or calculus) is hardened plaque that forms on the teeth due to mineralization and accumulates over time. These extrinsic stains can often be polished or cleaned away, whereas intrinsic stains like dental fluorosis require more complex cosmetic solutions to address.