A 23-year-old male medical student fell asleep in his chair with Netter’s Atlas wedged into his axilla. When he awoke in the morning, he was unable to extend the forearm, wrist, or fingers. Movements of the ipsilateral shoulder joint appear to be normal. Which of the following nerves was most likely compressed, producing the symptoms described?

**Core Concept**
Prolonged compression of a specific nerve in the upper limb can lead to muscle weakness and paralysis of the affected muscles. This condition is often due to a positional or anatomical compromise of the nerve, which may be exacerbated by external pressure.

**Why the Correct Answer is Right**
The radial nerve is responsible for the innervation of the extensor muscles of the forearm, wrist, and fingers. Compression of the radial nerve can lead to the inability to extend the forearm, wrist, or fingers. The radial nerve runs along the radial groove of the humerus, and prolonged compression in this area can cause nerve damage, resulting in the symptoms described. The fact that movements of the ipsilateral shoulder joint appear to be normal suggests that the damage is localized to the forearm and hand.

**Why Each Wrong Option is Incorrect**
* **Option A:** The median nerve is primarily responsible for the innervation of the muscles of the anterior forearm and the thenar eminence, as well as sensation to the palmar aspect of the thumb, index, and middle fingers. Compression of the median nerve would not result in the inability to extend the forearm, wrist, or fingers.
* **Option B:** The ulnar nerve is responsible for the innervation of the flexor muscles of the forearm and the intrinsic muscles of the hand, as well as sensation to the little finger and half of the ring finger. Compression of the ulnar nerve would not result in the inability to extend the forearm, wrist, or fingers.
* **Option D:** The axillary nerve is primarily responsible for the innervation of the deltoid muscle and the teres minor muscle, as well as sensation to the skin overlying the deltoid muscle. Compression of the axillary nerve would not result in the inability to extend the forearm, wrist, or fingers.

**Clinical Pearl / High-Yield Fact**
The radial nerve is susceptible to compression due to its anatomical course along the radial groove of the humerus. This compression can be exacerbated by external pressure, such as that caused by a heavy object, like a textbook, wedged into the axilla.

**Correct Answer:** C. Radial nerve.

✓ Correct Answer: C. Radial nerve