A 28-year-old undergoes an excision biopsy of a lymph node in the posterior triangle of the neck. Postoperatively, he has difficulty shrugging the ipsilateral shoulder and weakness of shoulder abduction. The accessory nerve (CN XI) was injured. At which anatomical point does the accessory nerve typically enter the posterior triangle?

• A Midpoint of the anterior border of trapezius
• B Jugular foramen
• C Midpoint of the posterior border of sternocleidomastoid ✓
• D Anterior border of sternocleidomastoid at the level of the hyoid bone

Explanation

The accessory nerve exits the skull via the jugular foramen, descends obliquely through the sternocleidomastoid (which it supplies), and enters the posterior triangle at the midpoint of the posterior border of sternocleidomastoid. It then runs on the levator scapulae across the floor of the posterior triangle to reach the anterior border of trapezius. This midpoint is the classic Erb's point of the posterior triangle, a key landmark for nerve identification during lymph node biopsy.

Reference: Gray's Anatomy, 42nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.