**Core Concept**
Horner's syndrome is a neurologic disorder characterized by a disruption in the sympathetic nerves supplying the face and eye on one side of the body. This disruption leads to a constellation of symptoms and signs due to impaired sympathetic function.

**Why the Correct Answer is Right**
Exophthalmos, or bulging of the eye, is not a feature of Horner's syndrome. In fact, Horner's syndrome is often associated with enophthalmos, or sinking of the eye, due to the loss of sympathetic tone in the orbital smooth muscles. This loss of tone results in decreased eye volume and a sunken appearance. Horner's syndrome is also characterized by ptosis (drooping of the upper eyelid), miosis (pupil constriction), anhydrosis (reduced sweating), and sometimes, enophthalmos.

**Why Each Wrong Option is Incorrect**
**Option B:** Ptosis is actually a feature of Horner's syndrome due to the loss of sympathetic tone in the superior tarsal muscle, which leads to drooping of the upper eyelid.
**Option C:** Miosis is a characteristic feature of Horner's syndrome due to the loss of sympathetic tone in the dilator pupillae muscle, leading to pupil constriction.
**Option D:** Anhydrosis, or reduced sweating, is also a feature of Horner's syndrome due to the loss of sympathetic tone in the sweat glands of the face.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with suspected Horner's syndrome, look for signs of sympathetic disruption, including ptosis, miosis, and anhydrosis. Enophthalmos may also be present, but exophthalmos is not a feature of this condition.

**✓ Correct Answer: A. Exophthalmos**