A 45-year-old woman is admitted to the hospital with neck pain. A CT scan reveals a tumor in the left side of her oral cavity. The tumor and related tissues are removed with a radical neck surgical procedure. Two months postoperatively the patient’s left shoulder droops quite noticeably. Physical examination reveals distinct weakness in turning her head to the right and impairment of abduction of her left upper limb to the level of the shoulder. Which of the following structures was most likely injured during the radical neck surgery?

## **Core Concept**
The question tests knowledge of the anatomy and innervation of the muscles involved in shoulder movement and head rotation, specifically in the context of radical neck surgery. The symptoms described point towards damage to a specific nerve that controls these functions. The **spinal accessory nerve (cranial nerve XI)** is crucial for the innervation of the sternocleidomastoid and trapezius muscles.

## **Why the Correct Answer is Right**
The correct answer, , involves the spinal accessory nerve (cranial nerve XI), which is responsible for innervating two key muscles: the sternocleidomastoid and the trapezius. The trapezius muscle is essential for shoulder movements, including abduction of the upper limb to the level of the shoulder, and for elevating, depressing, and rotating the scapula. The sternocleidomastoid muscle plays a significant role in rotating the head to the opposite side and flexing the neck. Injury to the spinal accessory nerve during radical neck surgery would lead to weakness in turning the head to the right (since the left sternocleidomastoid is affected) and impairment in abduction of the left upper limb, as well as the observed drooping of the shoulder due to trapezius muscle weakness.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a structure related to the symptoms described. Without a specific structure mentioned, it's impossible to assess its relevance to the injury.
- **Option B:** This option is incorrect because, although it might relate to nerves in the neck, it does not directly correlate with the symptoms described (shoulder drooping and weakness in turning the head).
- **Option C:** This option is incorrect because it does not directly relate to the functions impaired (shoulder abduction and head rotation), which are specifically associated with the spinal accessory nerve.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **spinal accessory nerve (cranial nerve XI)** has a unique course, exiting the skull through the jugular foramen and then traveling downwards through the neck, making it susceptible to injury during radical neck dissections. Clinically, testing for spinal accessory nerve function involves assessing the strength of the trapezius and sternocleidomastoid muscles.

## **Correct Answer:** . spinal accessory nerve (cranial nerve XI)

✓ Correct Answer: C. Spinal accessory nerve