If it won't scrape off with your fingernail, it is NOT Candida (thrush). The fact that they are still pursuing treatment of thrush given this leaves me less than confident with them. It is most likely leucoplakia. If you are a smoker, often this will resolve within a few months of tobacco cessation. If you are not, there are other means to remove it, such as laser. Leucoplakia is a precancerous condition, but of the precancerous lesions, it is the least dangerous as only about 25% of them convert to malignancy. A small incisional biopsy will define without doubt what it is, but the problem with this approach is that there can be dysplasia randomly within the leucoplakia (which is movement of the cells to the dark side), and deserves more aggressive thinking. The question arises is to where in a good size patch of white do you biopsy? It is a shot in the dark, so they usually take several samples. They still could miss something serious with this technique.

Another option is to stain the patch with Toludine blue, which is attracted to the mitochondria in dysplasia and malignant cells. That would identify an area in the white patch worthy of biopsy. As the posts above mention the only sure solution is to remove the entire thing (luckily it is superficial and does not require deep incisions and heals quickly). This allows a microscopic examination of the cells to see what