Okay, let's tackle this question step by step. The patient received an electric shock and fell down, resulting in an inability to perform external rotation of the shoulder and arm movement. The key here is to identify the nerve involved.

First, the shoulder's external rotation is primarily controlled by the infraspinatus and teres minor muscles. These muscles are innervated by the axillary nerve. The axillary nerve also provides sensation to the skin over the shoulder. So, if the axillary nerve is damaged, the patient would lose external rotation and arm movement.

Now, electric shocks can cause nerve injuries, especially if the current passes through the body in a way that affects the brachial plexus. The axillary nerve is part of the brachial plexus and is vulnerable in certain positions, like when falling on an outstretched hand, leading to shoulder dislocation. However, in this case, the injury is due to an electric shock, which might cause a different kind of trauma.

Looking at the options, the correct diagnosis would be axillary nerve injury. Let's consider other nerves. The musculocutaneous nerve innervates the biceps and coracobrachialis, affecting elbow flexion and forearm supination, not shoulder external rotation. The radial nerve affects wrist extension and elbow extension. The suprascapular nerve is involved in shoulder abduction and internal rotation. The ulnar nerve is more about hand movements. So, the other options don't fit the symptoms described.

Clinical pearls: Remember that axillary nerve injury is classic after shoulder dislocation, but electric shocks can also cause such injuries. The key signs are loss of external rotation and shoulder abduction, along with sensory loss over the deltoid area.

**Core Concept**
The axillary nerve, a branch of the brachial plexus (C5-C6), innervates the deltoid (for abduction) and teres minor (assisting external rotation). Injury to this nerve impairs shoulder external rotation and abduction, often due to shoulder dislocation or trauma. Electric shock-induced trauma can also damage this nerve directly.

**Why the Correct Answer is Right**
Axillary nerve injury results in loss of external rotation (teres minor dysfunction) and shoulder abduction (deltoid weakness). The patient’s inability to move the arm suggests deltoid paralysis. Electric shock can cause direct neuropraxia or indirect trauma to the axillary nerve, which is vulnerable at the surgical neck of the humerus. Classic signs include "flat shoulder" on examination and sensory loss over the lateral shoulder.

**Why Each Wrong Option is Incorrect**
**Option A:** Musculocutaneous nerve injury affects elbow flexion (biceps) and forearm supination (supinator), not shoulder external rotation.
**Option B:** Radial nerve palsy causes wrist drop and extensor weakness, unrelated to shoulder movement.
**Option C:** Suprascapular nerve injury impairs abduction and depression of the scapula, not external rotation.
**Option D:** Ulnar nerve palsy affects intrinsic hand muscles and elbow flexion, not shoulder function.

**Clinical Pearl / High-Yield Fact**
Axillary nerve injury is a red flag for shoulder dislocation or traumatic events (e.g., electric shock). Remember the "axillary nerve triangle" (C5-C6 roots) and its anatomical course near

✓ Correct Answer: B. Posterior dislocation