**Horner's Syndrome Overview**
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Horner's syndrome is a combination of symptoms resulting from disruption of the sympathetic nerves supplying the eye. It is characterized by ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (absence of sweating) on the affected side of the face. The sympathetic pathway responsible for these symptoms involves the hypothalamus, brainstem, spinal cord, and cervical sympathetic nerves.

**Why the Correct Answer is Right**
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The correct answer to this question is **A. Enophthalmos**. Enophthalmos is the posterior displacement of the globe of the eye into the orbit, which is a characteristic feature of Horner's syndrome. This occurs due to paralysis of the superior tarsal muscle, which is innervated by the sympathetic nerves. The loss of sympathetic tone leads to a decrease in the volume of the orbit, resulting in enophthalmos.

**Why Each Wrong Option is Incorrect**
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* **Option B:** Miosis is a key feature of Horner's syndrome, resulting from the disruption of the sympathetic nerves that stimulate the dilator pupillae muscle.
* **Option C:** Ptosis is also a feature of Horner's syndrome, resulting from the paralysis of the superior tarsal muscle.
* **Option D:** Anhidrosis is a feature of Horner's syndrome, resulting from the disruption of the sympathetic nerves that innervate the sweat glands.

**Clinical Pearl / High-Yield Fact**
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Horner's syndrome can be a sign of various conditions, including carotid artery dissection, cervical spine trauma, and neuroblastoma. It is also a useful clinical sign in diagnosing conditions that affect the sympathetic nervous system.

**Correct Answer: A. Enophthalmos**