A patient presents with a large ulcerated lesion with mixed red and white areas covering the entire right buccal mucosa. The patient is a 55-year-old heavy smoker and alcohol user. What is the most likely diagnosis?

Oral squamous cell carcinoma (OSCC) is the most likely diagnosis for a large ulcerated, mixed red-white (erythroleukoplakic) lesion of the buccal mucosa in a middle-aged heavy smoker and alcohol user. OSCC accounts for approximately 90% of oral malignancies. The combination of tobacco use and alcohol consumption multiplies the risk dramatically. Features suggesting malignancy include: ulceration, mixed red-white appearance, induration on palpation, large size, and a history of tobacco/alcohol use. Any such lesion requires immediate incisional biopsy. Pleomorphic adenoma is a benign salivary gland tumor, typically painless and firm, not ulcerated.