**Question:** Horner syndrome causes all of the following signs, EXCEPT:

A. Ptosis (drooping of the upper eyelid)
B. Miosis (narrowing of the pupil)
C. Anhidrosis (inability to sweat)
D. Enophthalmos (eye sinking)

**Core Concept:**
Horner syndrome is a clinical condition characterized by a constellation of symptoms resulting from the disruption of the sympathetic innervation to the affected eye. It is typically caused by damage to the cervical sympathetic chain, often due to a tumor, stroke, or vascular injury. The affected eye may present with pupillary changes, such as miosis (narrowing of the pupil) and ptosis (drooping of the upper eyelid). However, it is expected to be associated with other signs as well, like anhidrosis (inability to sweat) and enophthalmos (eye sinking).

**Why the Correct Answer is Right:**
The correct answer, **D. Enophthalmos (eye sinking)**, is excluded from the signs of Horner syndrome due to its correlation with the opposite condition, known as Marcus Gunn pupil (also called the "Moon Face" or "Puff of Air" sign). Enophthalmos occurs when there is an increase in the volume of tissue in the orbit due to the lack of contraction of the levator palpebrae superioris muscle, causing the eye to sink within its bony orbit. On the other hand, Horner syndrome results from the loss of sympathetic innervation to the affected eye, leading to miosis, ptosis, and anhidrosis but not enophthalmos.

**Why Each Wrong Option is Incorrect:**
A. Ptosis (drooping of the upper eyelid) and B. Miosis (narrowing of the pupil) are involved in the pathogenesis of Horner syndrome. These symptoms result from the loss of sympathetic innervation to the affected eye, causing the smooth muscle of the levator palpebrae superioris to be inactive.

C. Anhidrosis (inability to sweat) is another key sign of Horner syndrome, reflecting the disruption of the sympathetic innervation to the affected eye, leading to reduced sweating.

**Clinical Pearl:**
While the classical triad of Horner syndrome consists of miosis, ptosis, and anhidrosis, it is essential to understand that these symptoms are not exclusive to the condition. Enophthalmos, also known as Marcus Gunn pupil or the "Puff of Air" sign, is a characteristic feature of the opposite condition, Marcus Gunn pupil (MGP). MGP is a result of the loss of parasympathetic innervation to the affected eye, leading to dilation of the pupil, when compared to the other eye. This differential diagnosis is crucial for clinicians to distinguish between Horner syndrome and Marcus Gunn pupil, ensuring appropriate diagnosis and management.

**Core Concepts:**
- Horner syndrome: loss of sympathetic innervation to the affected eye resulting in miosis (narrowing of the pupil), ptosis (drooping of the upper