A 6-year-old child for a 3rd day presents with body temperature up to 38-39°C and painful swallowing. Objectively on the mucosa of the palatine arches, tonsils and a part of the soft palate there are several erosions up to 2 mm in diameter. Tire erosions are situated on the hyperemic mucosa and covered in whitish plaque. Submandibular lymph nodes are enlarged and painless on palpation. What is the most likely diagnosis?