A 16-year-old young man complains of temperature increase up to 38.7°C, pain when eating and swallowing, foul acrid smell from his mouth. Lymph nodes, especially cervical ones, are enlarged, mobile, and painless. Objectively the patient presents with generalized hyperemia of the oral mucosa, multiple petechiae, necrotic spots, and profuse coating of the anterior pharynx. Blood test: increased ESR, marked leukocytosis, monocytosis, atypical mononuclear cells, thrombocytopenia. What is the most likely diagnosis?