## **Core Concept**
Muller's muscle, also known as the superior tarsal muscle, is a smooth muscle that plays a crucial role in eyelid elevation. It is involved in the regulation of eyelid movement, particularly in maintaining the position of the eyelid.

## **Why the Correct Answer is Right**
The correct answer, **C. sympathetic nervous system**, is right because Muller's muscle receives sympathetic innervation. This muscle is unique because it is a smooth muscle in the eyelid that is innervated by the sympathetic nervous system, which allows it to function involuntarily. The sympathetic nerves that supply Muller's muscle are responsible for its contraction, which aids in eyelid elevation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Muller's muscle is not supplied by the facial nerve, which primarily controls the muscles of facial expression, including the orbicularis oculi but not the smooth muscle of the eyelid.
- **Option B:** This option is incorrect as it refers to the oculomotor nerve (cranial nerve III), which controls some of the extraocular muscles and the levator palpebrae superioris muscle but not Muller's muscle directly.
- **Option D:** This option is incorrect because the trigeminal nerve primarily provides sensory innervation to the face and motor innervation to muscles of mastication, not to Muller's muscle.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that Horner's syndrome, which results from disruption of the sympathetic nerves supplying the eye, can lead to ptosis (drooping eyelid) due to the loss of sympathetic innervation to Muller's muscle. This highlights the importance of sympathetic innervation in eyelid elevation.

## **Correct Answer:** . **C. sympathetic nervous system**