A 39-year-old patient, who works in a printing house, complains of burning pain, metallic taste in the mouth and bad breath. Objectively: the skin is sallow in colour. The oral mucosa has black-and-blue spots. The gums are hyperemic and swollen. The edge of the gums has black-and-blue band. The palate is of yellow shade. Intoxication symptoms are observed. What is the most probable diagnosis?