**Question:** A man with Pancoast tumor developed Horner's syndrome. All of the following are the features of Horner's syndrome, EXCEPT:

A. Ptosis (drooping of the upper eyelid)
B. Anhidrosis (lack of sweating)
C. Miosis (narrowing of the pupil)
D. Enophthalmos (protrusion of the eye)

**Correct Answer:** D. Enophthalmos (protrusion of the eye)

**Core Concept:**
Horner's syndrome is a clinical syndrome characterized by a triad of symptoms: ptosis, miosis, and anhidrosis. The syndrome is caused by interruption or damage to the sympathetic nervous system in the neck, usually due to a Pancoast tumor (a pulmonary tumor involving the superior pulmonary vein or the brachial plexus). In a Pancoast tumor patient, the symptoms are typically related to the effects of the tumor on the sympathetic nervous system in the neck.

**Why the Correct Answer is Right:** Enophthalmos is not a feature of Horner's syndrome. Enophthalmos refers to the protrusion of the eye, which is caused by a decrease in orbital contents due to compression or destruction of the medial wall of the orbit. In contrast, Horner's syndrome results from a disruption in the sympathetic chain, causing ptosis, miosis, and anhidrosis. Enophthalmos is not part of the clinical presentation of Horner's syndrome.

**Why Each Wrong Option is Incorrect:**

A. Ptosis (drooping of the upper eyelid) - This is a correct feature of Horner's syndrome, as it results from the disruption of the sympathetic chain.

B. Anhidrosis (lack of sweating) - This is also a correct feature of Horner's syndrome, as it results from the disruption of the sympathetic chain and its effects on eccrine glands.

C. Miosis (narrowing of the pupil) - This is a correct feature of Horner's syndrome, as it results from a disruption in the sympathetic chain and its effects on the pupillary constriction mechanism.

**Clinical Pearls:**

1. Horner's syndrome is a clinical sign of a sympathetic nervous system dysfunction.
2. Although the syndrome can be caused by various conditions, in the context of a Pancoast tumor, it is important for students to understand the specific effects of a Pancoast tumor on the sympathetic chain and the resulting clinical features.
3. Enophthalmos is not part of the Horner's syndrome clinical picture and should not be confused with the syndrome. Instead, it is a separate entity that requires a different evaluation and treatment approach.