Parents of an 8-year-old child complain about a painful formation in the child's oral cavity that obstructs food intake. The same complaints were registered two years ago. Mucous membrane of lateral tongue surface is hyperemic and edematic. There is an oval erosion over 0,7 cm large covered with yellow greyish deposit. Erosion edges are hyperemic and painful on palpation. The child has a history of chronic cholecystocholangitis. What is the most likely diagnosis?