Following a penetrating shoulder injury, a patient cannot abduct the arm beyond 15 degrees, has loss of sensation over the lateral arm (regimental badge area), and wasting of the deltoid. The posterior cord of the brachial plexus is disrupted. Which nerve is injured and at what spinal levels does it originate?

• A Musculocutaneous nerve (C5, C6, C7) from lateral cord
• B Radial nerve (C5-T1) from posterior cord
• C Suprascapular nerve (C5, C6) from upper trunk
• D Axillary nerve (C5, C6) from posterior cord ✓

Explanation

The axillary nerve (C5, C6) arises from the posterior cord and winds around the surgical neck of the humerus. It supplies the deltoid (abduction 15–90 degrees), teres minor, and provides the upper lateral cutaneous nerve of arm to the lateral arm (regimental badge area). The first 15 degrees of abduction is initiated by supraspinatus (suprascapular nerve, C5, C6) from the upper trunk. Radial nerve injury produces wrist drop, not isolated deltoid wasting. The musculocutaneous nerve arises from the lateral cord and is not the primary abductor nerve.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.