Final answer:
The most common cause of unilateral proptosis in adults is Graves' Disease, a thyroid-related autoimmune condition that can lead to inflammation and bulging of the eyeball.
Step-by-step explanation:
The most common cause of unilateral proptosis in adults is related to issues within the thyroid system. Specifically, a condition called Graves' Disease is frequently associated with this symptom. Patients with Graves' Disease may exhibit proptosis and lid retraction due to the autoimmune process that affects the thyroid and can also involve the tissues around the eyes.
Graves' Disease leads to inflammation and other changes in the eye muscles and fat, causing the eyeball to protrude (proptosis), creating a characteristic bulging appearance. It is different from conditions that affect the visual field such as a pituitary gland growth causing bilateral hemianopia, where the individual's peripheral vision is compromised, but this affects both eyes and does not result in proptosis.