A 16-year-old young man complains of a temperature increase up to 38.7°C, pain when eating and swallowing, foul sharp smell from his mouth. The lymph nodes, especially the 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?