According to the mother, a 5-year-old child complains about pain during swallowing, weakness, body temperature rise upt to 39,5°C, swelling of submental lymph nodes. Objectively: the child's condition is grave, body temperature is 38,8°C. Mucous membrane of the oral cavity is markedly hyperaemic and edematic with hemorrhages and ulcerations. Pharynx is markedly hyperemic, lacunae are enlarged and have necrosis areas. Regional, cervical, occipital lymph nodes are painful, enlarged and dense. What is the most likely diagnosis?