A 57-year-old patient complains about dryness and burning of the lateral surface of her tongue. These sensations disappear during eating. She noted such sensations three months ago. She has a history of gastritis with reduced secretory function. Objectively: mucous membrane of tongue and oral cavity has no peculiarities. The back of tongue has thin white coating. Regional lymph nodes are unpalpable. Oral cavity is sanitized. What is the most likely diagnosis?