A 28-year-old male presents with a stab wound to the axilla. Clinical examination reveals loss of sensation over the lateral shoulder (regimental badge area) and inability to abduct the arm at the shoulder. All of the following nerves are likely to be intact EXCEPT:

Explanation

## Clinical Presentation of Axillary Nerve Injury ### Clinical Findings in This Case **Key Point:** The patient has two cardinal signs of axillary nerve injury: 1. **Loss of sensation over the lateral shoulder** (regimental badge area) — supplied by the axillary nerve's sensory branch 2. **Loss of shoulder abduction** — due to paralysis of the deltoid muscle (primary motor innervation of axillary nerve) ### Axillary Nerve Anatomy and Function | Feature | Details | |---|---| | **Origin** | Posterior cord of brachial plexus (C5, C6) | | **Course** | Passes through the quadrangular space with posterior circumflex humeral artery | | **Motor supply** | Deltoid (abduction 15–90°), teres minor | | **Sensory supply** | Lateral shoulder (regimental badge area) | | **Common injury sites** | Shoulder dislocation, axillary artery puncture, stab wounds | **High-Yield:** The axillary nerve is the ONLY nerve that supplies the deltoid. Loss of abduction at the shoulder is pathognomonic for axillary nerve injury. ### Why Other Nerves Are Intact | Nerve | Motor Function | Status in This Case | |---|---|---| | **Musculocutaneous** | Elbow flexion (biceps, brachialis) | Intact — elbow flexion preserved | | **Radial** | Wrist/finger extension, elbow extension | Intact — wrist extension preserved | | **Ulnar** | Intrinsic hand muscles, wrist flexion | Intact — hand grip preserved | | **Axillary** | **Shoulder abduction (deltoid)** | **INJURED** | **Clinical Pearl:** A stab wound to the axilla is a classic mechanism for axillary nerve injury because the nerve passes through the quadrangular space (bounded by teres major, teres minor, long head of triceps, and humerus) — a vulnerable anatomical bottleneck. ### Differential Diagnosis of Shoulder Abduction Loss ```mermaid flowchart TD A[Loss of shoulder abduction]:::outcome --> B{Nerve injury vs muscle injury?}:::decision B -->|Nerve| C{Which nerve?}:::decision C -->|Axillary nerve| D[Deltoid paralysis + sensory loss lateral shoulder]:::action C -->|Suprascapular nerve| E[Supraspinatus paralysis only]:::action B -->|Muscle| F[Rotator cuff tear or deltoid rupture]:::action D --> G[Stab wound, shoulder dislocation, axillary artery puncture]:::outcome ```