A 31-year-old male patient complains of dryness and burning of tongue back that appeared for about a week ago and get worse when he eats irritating food. The patient has a history of recent pneumonia. He had been treated in the in-patient hospital for 2 weeks, the treatment program included antibiotics. Now he doesn't take any drugs. Objectively: mucous membrane of the oral cavity is hyperemic, dry, glossy. Tongue back and palate have greyish-white plicae that can be easily removed. Threads of saliva trail behind the spatula. What is the most likely diagnosis?