A 50-year-old man with a painless white patch on the tongue may be suffering from which condition?

The presence of a painless white patch on the tongue is most characteristic of oral leukoplakia. This condition is often associated with keratinization of the oral mucosa and can result from chronic irritation, such as from tobacco use or rough teeth. Oral leukoplakia lesions are generally painless and may present as white patches or plaques that cannot be easily scraped off, distinguishing them from other conditions.

While oral candidiasis is typically associated with painful, white patches that can be scraped away, and oral lichen planus can present with white lesions but usually also has an itchy or burning sensation along with other characteristic appearances, these features are not consistent with the condition described in the question. Similarly, squamous cell carcinoma of the tongue can present with white patches but often comes with other concerning symptoms, such as pain, ulcers, or a change in the appearance of the tongue, making leukoplakia the most fitting diagnosis in this context.