Which of the following is a classic clinical sign of Horner's syndrome?

Enophthalmos is a classic clinical sign of Horner's syndrome. This condition occurs due to a disruption of the sympathetic nerve pathway to the eye, leading to various ocular manifestations. Horner’s syndrome causes the affected eye to appear sunken (enophthalmic) because of the decrease in intraocular pressure and the effect on the smooth muscles that control the position of the eyeball.

The other signs mentioned in the options represent different ocular conditions not related to Horner's syndrome. Buphthalmos refers to an abnormal enlargement of the eye typically seen in glaucoma, proptosis is the forward displacement of the eyeball often associated with trauma or other conditions, and microphthalmos is a developmental anomaly where the eye is abnormally small. Understanding these distinctions helps clarify why enophthalmos is specifically associated with Horner's syndrome, as it reflects the underlying physiological changes caused by sympathetic nerve dysfunction.