A 35-year-old male presents to the emergency department following a fall from a ladder with a suspected anterior shoulder dislocation. On examination before reduction, the structure marked **A** (axillary nerve) is tested. Which of the following clinical findings would indicate injury to the structure marked **A**?

Explanation

## Why option 1 is correct The axillary nerve (C5-C6) wraps around the surgical neck of the humerus through the quadrangular space and innervates the deltoid muscle and provides sensory supply to the lateral shoulder region. Injury to the axillary nerve results in two cardinal findings: (1) **motor loss** — deltoid paralysis causing loss of shoulder abduction beyond 15° (the initial 15° of abduction is performed by the supraspinatus), and (2) **sensory loss** over the "regimental badge area" — the lateral deltoid and upper outer arm. This is the classic presentation of axillary nerve injury following anterior shoulder dislocation, which is the most common cause of axillary nerve injury. Testing deltoid sensation before reduction is medico-legally critical to document that the injury was caused by the dislocation itself, not the reduction maneuver (Gray's Anatomy 42e Ch 47). ## Why each distractor is wrong - **Option 2 (medial forearm sensation, elbow flexion loss)**: This describes injury to the musculocutaneous nerve (C5-C6-C7), which innervates the biceps and brachialis muscles and provides sensory supply to the lateral forearm, not the axillary nerve. - **Option 3 (dorsal first web space, thumb extension loss)**: This describes injury to the posterior interosseous nerve (branch of radial nerve, C7-C8), which innervates the extensor muscles of the hand and provides sensory supply to the dorsal first web space, not the axillary nerve. - **Option 4 (medial arm sensation, shoulder adduction loss)**: This describes injury to the ulnar nerve (C8-T1) or thoracodorsal nerve, which innervate the adductor muscles and provide sensory supply to the medial arm, not the axillary nerve. **High-Yield:** Axillary nerve injury → **deltoid paralysis** (loss of abduction >15°) + **regimental badge sensory loss** (lateral deltoid/upper outer arm); most common cause is anterior shoulder dislocation; always test sensation BEFORE reduction. [cite:Gray's Anatomy 42e Ch 47]