Final answer:
Leukoplakia caused by Sanguinaria-containing toothpaste is most likely located in the buccal mucosa, which lines the inside of the cheeks.
Step-by-step explanation:
If the Leukoplakia was caused by patients using Sanguinaria-containing toothpaste, the leukoplakia is most likely located in the buccal mucosa in the mouth. Toothpaste is typically applied to the teeth and gums during brushing, and thus it primarily comes into contact with the buccal mucosa, which lines the inside of the cheeks. Leukoplakia occurring in this area is often associated with the use of such dental products, especially when they contain potentially irritating substances like Sanguinaria.