Which of the following nerves is derived from the posterior cord of the brachial plexus?

Explanation

## Posterior Cord and Its Branches ### Posterior Cord Anatomy **Key Point:** The posterior cord of the brachial plexus is formed by the union of all three trunks (upper, middle, and lower) at the level of the axilla. It is the only cord that receives contributions from all three trunks. ### Branches of the Posterior Cord | Branch | Primary Function | Innervation | |--------|------------------|-------------| | **Radial nerve** | Arm and forearm extension; wrist/finger extension | Triceps, extensor carpi radialis, extensor carpi ulnaris, extensor digitorum, posterior forearm | | **Axillary nerve** | Shoulder abduction and external rotation | Deltoid, teres minor | | **Subscapular nerves** (upper & lower) | Scapular stabilization | Subscapularis, teres major, latissimus dorsi | | **Thoracodorsal nerve** | Scapular/arm depression | Latissimus dorsi | **High-Yield:** The radial nerve is the largest and most clinically important branch of the posterior cord. ### Mnemonic: Cord-Branch Relationships **"MARLIN"** — Musculocutaneous (Lateral cord), Axillary & Radial (Posterior cord), Median & Ulnar (Medial cord + Lateral cord). ### Clinical Pearl Radial nerve injuries (e.g., from mid-shaft humerus fractures or Saturday night palsy) cause **wrist drop** and loss of finger extension — a hallmark sign of posterior cord / radial nerve compromise.