A 7-year-old child complains of fever up to 38,3°C, headache, sore throat and general weakness. Objectively: the mucosa of the soft palate, tonsils and palatine arches is edematous and hyperemic, there are numerous small painful erosions with red bottom. The erosions are not covered with any plaque. The submandibular lymph nodes are enlarged, painful on palpation. What is the most likely diagnosis?