The parents of a 6-year-old child complain that their child for the last three days had body temperature of 39°C, weakness, loss of appetite, and pain during swallowing. Objectively, the mucosa of the tonsils, palatine arches, retromolar space, and posterior and lateral pharyngeal walls is swollen and has a dirty-gray coating that is tightly attached to the underlying tissues. The coating is very difficult to remove. When removed, it exposes a bleeding surface. The submandibular lymph nodes are enlarged and painful. What is the most likely provisional diagnosis?