A 45-year-old man undergoes excisional biopsy of a suspicious lymph node in the posterior triangle of the neck for staging of suspected lymphoma. During the procedure, the structure marked **A** is inadvertently transected. Which of the following clinical findings would you expect as a direct consequence of this injury?

Explanation

## Why "Inability to shrug the shoulders and winging of the scapula on lateral arm raise" is right The structure marked **A** is the spinal accessory nerve (CN XI), which crosses the posterior triangle superficially, lying just deep to the investing layer of cervical fascia. This anatomical position makes it highly vulnerable to injury during lymph node biopsy, radical neck dissection, or penetrating trauma. CN XI innervates the trapezius muscle, which is essential for shoulder elevation (shrugging) and upward scapular rotation. Transection of CN XI results in paralysis of the trapezius, causing characteristic shoulder droop, inability to shrug, weakness in abducting the arm above 90°, and winging of the scapula on lateral arm raise. This is a classic and clinically significant complication of posterior triangle surgery (Gray's Anatomy 42e Ch 30; Bailey & Love 28e). ## Why each distractor is wrong - **Loss of sensation over the lateral neck and ear lobe**: This describes injury to the greater auricular nerve or transverse cervical nerve (sensory branches from the cervical plexus at Erb's point), not CN XI. CN XI is purely motor to the trapezius and sternocleidomastoid. - **Weakness in turning the head to the contralateral side with intact shoulder function**: While CN XI does innervate the sternocleidomastoid (causing weakness in head turning), the question asks for the direct consequence of CN XI injury, which primarily manifests as trapezius paralysis with shoulder dysfunction. This option incompletely captures the motor deficit. - **Paralysis of the diaphragm with preserved upper limb sensation**: This describes phrenic nerve injury (C3, C4, C5), not CN XI. The phrenic nerve is located more medially and anteriorly in the neck, not in the posterior triangle. **High-Yield:** CN XI is the most vulnerable cranial nerve in the posterior triangle—always identify and protect it during posterior triangle surgery to avoid trapezius paralysis and scapular winging. [cite: Gray's Anatomy 42e Ch 30; Bailey & Love 28e]