## **Core Concept**
The sternocleidomastoid muscle is one of the major muscles in the neck, playing a crucial role in movements of the head and neck. It is innervated by a specific cranial nerve that controls its motor function.

## **Why the Correct Answer is Right**
The sternocleidomastoid muscle is supplied by the **Accessory nerve (XIth cranial nerve)**, also known as the spinal accessory nerve. This nerve has a cranial root and a spinal root; the spinal root is responsible for innervating the sternocleidomastoid and trapezius muscles. The accessory nerve provides the motor innervation necessary for the muscle's functions, including rotation of the head to the opposite side and flexion of the neck.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Mandibular division of the Trigeminal nerve (V3)** primarily controls muscles of mastication (chewing), including the medial pterygoid, tensor tympani, tensor veli palatini, and the muscles of the jaw (masseter, temporalis, and digastric). It does not innervate the sternocleidomastoid muscle.
- **Option B:** The **Facial nerve (VIIth cranial nerve)** is primarily responsible for controlling the muscles of facial expression and transmitting taste from the anterior two-thirds of the tongue. It does not supply the sternocleidomastoid muscle.
- **Option C:** The **Ansa Cervicalis** is a loop of nerves from the cervical plexus that supplies infrahyoid muscles (sternohyoid, sternothyroid, and omohyoid) but not the sternocleidomastoid muscle.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that damage to the accessory nerve (XIth cranial nerve) can result in weakness or paralysis of the sternocleidomastoid and trapezius muscles. This can lead to difficulties in rotating the head and stabilizing the scapula. A classic clinical test for assessing accessory nerve function is the **trapezius test**, where the patient's ability to shrug their shoulders against resistance is evaluated.

## **Correct Answer:** D. Accessory nerve (XIth cranial nerve).