Final answer:
A white lesion on the retromolar pad opposing a non-functional molar is most likely alveolar ridge keratosis, commonly related to chronic local irritation. Other conditions such as candidiasis, lichen planus, or squamous cell carcinoma have different presentations and are less likely in this scenario.
Step-by-step explanation:
The most likely diagnosis of a white lesion on the retromolar pad opposing a non-functional molar is alveolar ridge keratosis. Alveolar ridge keratosis is a benign lesion that typically appears due to chronic irritation, such as the friction or pressure created by an opposing tooth or dental appliance. In this context, because the molar is non-functional, it may contribute to the development of such a lesion through repetitive trauma. While other options such as candidiasis, lichen planus, and squamous cell carcinoma should be considered, they are less likely given the described scenario. Candidiasis, also known as thrush, typically presents as white patches and can affect the mouth but is associated with immunocompromised states and does not usually develop solely from mechanical irritation. Lichen planus and squamous cell carcinoma have different clinical presentations and risk factors.