A 55-year-old woman complains of ulcers on the mucous membrane of oral cavity, pain during eating and talking. She fell abruptly ill over a month ago. Objectively: unchanged mucous membrane of her gums, soft palate and palatine arches has big bright-red erosions on it. Intact mucous membrane peels easily off when slightly rubbed and this results in erosions and small hemorrhages. What is the leading diagnostic symptom in the differential disease diagnostics?