A 54-year-old woman complains of a neoplasm on her lower lip that appeared 6 months ago. During the last 20 days, it started sharply increasing in size. Objectively, on the vermilion border of the lower lip there is a round gray-red node that protrudes by 0.5 cm above the underlying tissues, is demarcated, and has a funnel-shaped indent in its center, filled with keratinized masses. The node is dense, mobile, and painless to palpation. What is the most likely diagnosis in this case?