A 10-year-old child complains of sore throat, cough, fever (up to 38°C). These presentations turned up 2 days ago. Objectively: acute catarrhal stomatitis is present. Tonsils are swollen, hyperemic, covered with yellow-gray friable film which can be easily removed. Submandibular and cervical lymph nodes are significantly enlarged, painful on palpation. Laboratory analysis revealed leuko- and monocytosis. What is the most likely diagnosis?