## **Core Concept**
Enophthalmos refers to the posterior displacement or sinking of the eyeball into the orbit. This condition can occur due to various causes, including trauma, tumors, or neurological disorders. In the context of Horner's syndrome, enophthalmos is a notable but uncommon feature.

## **Why the Correct Answer is Right**
The correct answer involves understanding the innervation responsible for the position and movement of the eyeball. The **superior tarsal muscle**, also known as Müller's muscle, plays a crucial role in eyelid elevation and is innervated by the sympathetic nervous system. However, enophthalmos in Horner's syndrome is primarily attributed to the relaxation of **orbital smooth muscles**, particularly the **inferior tarsal muscle** and the **capsulopalpebral muscle**, which are sympathetically innervated. When these muscles are denervated, as in Horner's syndrome, the eyeball can sink posteriorly into the orbit due to the loss of sympathetic tone.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the muscles listed are not primarily responsible for the phenomenon of enophthalmos in Horner's syndrome.
- **Option B:** This option is incorrect because, although Müller's muscle is involved in eyelid elevation and its dysfunction leads to ptosis in Horner's syndrome, it is not directly responsible for enophthalmos.
- **Option D:** This option is incorrect because the primary issue in enophthalmos related to Horner's syndrome is not with the nerves controlling extraocular muscle movement but with the sympathetic innervation of orbital smooth muscles.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Horner's syndrome classically presents with ptosis, miosis, and anhidrosis. Enophthalmos, though less commonly emphasized, can also be a feature. The ptosis in Horner's syndrome is typically mild and can be distinguished from ptosis due to other causes by the presence of the other signs of Horner's syndrome.

## **Correct Answer:** C.