A 2,5-year-old child has fever up to 38,5°C, low appetite, rash inside the oral cavity. The disease onset was 3 days ago. Objectively: the skin of the perioral area is covered in scarce vesicles with clear content. Within the oral cavity on the buccal and lingual mucosa there are sharply painful erosions, 2-3 mm in size, with white coating and hyperemic crown. The gums are swollen, hyperemic. The submandibular lymph nodes are enlarged, painful on palpation. Make the diagnosis: