A 60-year-old woman complains of a pain in the oral cavity that increases during eating. She has a history of ischemic heart disease. Objectively, her oral mucosa is pale pink, without visible pathological changes. On the lateral surface of the tongue there is an ulcer 0.7 cm in diameter. The ulcer has clear and even margins, is covered in a gray coating, and is painful on palpation. In the surrounding tissues there is no inflammatory reaction. Make the correct diagnosis: