Final answer:
The most likely diagnosis for a patient with a round ulcer in the mouth is a canker sore.
Step-by-step explanation:
The most likely diagnosis for a patient presenting with a round ulcer in their mouth that is yellow-grey and has a red halo on the buccal mucosa is a canker sore. Canker sores are open and painful mouth ulcers that are usually white or yellowish, surrounded by red, inflamed soft tissue. They are not contagious and usually heal on their own within a week or two.
A canker sore, also known as aphthous ulcer, is a small, shallow, painful sore or lesion that can form inside your mouth. These sores are different from cold sores, which are caused by the herpes simplex virus and typically appear on the outer lip.