A patient with a history of betel nut chewing presents with a brown cluster lesion on the buccal mucosa near teeth #18 and #19. What is the MOST likely diagnosis?

Oral Submucous Fibrosis (OSMF) is a chronic, progressive, potentially malignant disorder strongly associated with betel nut (areca nut) chewing. Arecoline (an alkaloid in betel nut) stimulates fibroblasts to overproduce collagen. Clinically it presents as progressive fibrosis of the oral mucosa causing trismus, burning sensation, blanching, and palpable fibrous bands. Brownish discoloration of the mucosa is common due to the nut staining. It has ~8% malignant transformation rate.