A 47-year-old man complains of pain and swelling in his left submandibular region and pain during eating. Objectively, the mouth opening is unrestricted, mucosa of the left sublingual fold is hyperemic, the excretory duct of the submandibular salivary gland produces a small amount of turbid saliva. X-ray of the mouth floor shows an oval shadow 1 cm in size. Make the diagnosis: