A 12-year-old child complains about sore throat, headache, body temperature rise up to 38,5°C, rhinitis, cough in summer period. Objectively: mucous membrane of oral cavity is hyperemic, edematic. There are 10-15 erosions up to 0,5 mm large on the palate and palatine arches, that aren't covered with deposit and have red floor. Regional lymph nodes are enlarged and painful on palpation. What is the most likely diagnosis?