## **Core Concept**
The dilator pupillae muscle is responsible for dilating the pupil. It is part of the iris and plays a crucial role in regulating the amount of light that enters the eye. The muscle is innervated by the autonomic nervous system.

## **Why the Correct Answer is Right**
The dilator pupillae muscle is supplied by the **sympathetic nervous system**. Specifically, it receives its innervation from the superior cervical ganglion via the postganglionic sympathetic fibers. When stimulated, these fibers cause the dilator pupillae muscle to contract, leading to pupil dilation. This is part of the body's 'fight or flight' response.

## **Why Each Wrong Option is Incorrect**
* **Option A:** The parasympathetic nervous system is primarily involved in the constriction of the pupil through the sphincter pupillae muscle, not dilation.
* **Option B:** While the oculomotor nerve (cranial nerve III) does carry parasympathetic fibers that innervate the sphincter pupillae muscle, it is not directly responsible for the innervation of the dilator pupillae muscle.
* **Option C:** The trigeminal nerve (cranial nerve V) is involved in sensory innervation of the face and motor innervation of muscles of mastication, not in the innervation of the dilator pupillae.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is Horner's syndrome, which results from disruption of the sympathetic pathway to the eye, leading to ptosis, miosis (pupil constriction), and anhidrosis. This highlights the importance of sympathetic innervation to the dilator pupillae muscle.

## **Correct Answer:** .