A 45-year-old male patient has been admitted to the maxillofacial department with complaints of general weakness, pain and swelling in the left submandibular region, elevation of body temperature up to 39°C. He has a history of frequent pain in the 36 mandibular tooth on the left. The face is asymmetric due to the swelling of the left submandibular region, where a dense infiltrate is palpable; the overlying skin is hyperemic, doesn't make a fold. There is a deep carious cavity in the 36 tooth, its percussion causes pain response; the surrounding mucosa is hyperemic, edematous. What is the most likely diagnosis?