A 7-year-old boy falls from a tree house and is brought to the emergency depament of a local hospital. On examination, he has weakness in rotating his arm laterally because of an injury of a nerve. Which of the following conditions is most likely to cause a loss of this nerve function?
**Core Concept**
The scenario describes a patient with a specific nerve injury leading to weakness in rotating the arm laterally. This symptom is associated with the radial nerve, which is responsible for controlling the muscles involved in elbow extension and forearm supination (rotation). The radial nerve is a branch of the brachial plexus, a complex network of nerves that innervate the upper limb.
**Why the Correct Answer is Right**
The radial nerve is prone to injury due to its anatomical course, particularly at the level of the spiral groove of the humerus. This location makes it susceptible to compression or trauma, leading to radial nerve palsy. In this case, the patient's weakness in rotating his arm laterally is consistent with a radial nerve injury, which affects the function of the supinator muscle. The supinator muscle is responsible for forearm supination, and its dysfunction leads to difficulty in rotating the arm laterally.
**Why Each Wrong Option is Incorrect**
* **Option A:** The ulnar nerve is responsible for controlling the muscles involved in forearm flexion and wrist extension. An injury to the ulnar nerve would not cause weakness in rotating the arm laterally.
* **Option B:** The median nerve is responsible for controlling the muscles involved in wrist flexion and thumb opposition. An injury to the median nerve would not cause weakness in rotating the arm laterally.
* **Option D:** The axillary nerve is responsible for controlling the muscles involved in shoulder abduction and external rotation. An injury to the axillary nerve would not cause weakness in rotating the arm laterally.
**Clinical Pearl / High-Yield Fact**
The radial nerve is often injured in the spiral groove of the humerus, particularly in patients who fall onto an outstretched hand or suffer a direct blow to the arm. This type of injury can lead to radial nerve palsy, characterized by weakness in elbow extension and forearm supination.
**Correct Answer:** C. Radial nerve palsy