A 54-year-old woman complains of a neoplasm on her lower lip that appeared 6 month ago. Twenty days ago, it suddenly started growing rapidly. Objectively, on the vermillion border of the lower lip there is a round red-gray node that rises 0.5 cm above the underlaying tissues. It is distinguished from the surrounding tissues and has a funnelform depression in its center, filled with keratinized masses. On palpation, the nodule is dense, mobile, and painless. What is the most likely diagnosis?