## **Core Concept**
The question tests knowledge of peripheral nerve injuries, specifically those affecting the upper limb. The **radial nerve** is primarily responsible for wrist and finger extension. Understanding the motor functions of different nerves in the upper limb is crucial for diagnosing nerve injuries.

## **Why the Correct Answer is Right**
The radial nerve (**C. Radial nerve**) is responsible for the extension of the wrist and fingers through its innervation of the extensor muscles in the forearm, such as the extensor carpi radialis brevis and extensor digitorum. However, the **extensor carpi radialis longus**, which is involved in wrist extension, is innervated proximal to the typical point of injury of the radial nerve. The ability to extend the elbow is primarily due to the **triceps brachii**, which is also innervated by the radial nerve but more proximally. Therefore, an injury to the radial nerve that spares the branches to the extensor carpi radialis longus and the triceps brachii could result in the inability to extend fingers but still allow wrist and elbow extension.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **median nerve** primarily controls thumb opposition, flexion of the thumb, index, and middle fingers, and sensation to the palmar surface of the thumb, index, middle finger, and the radial half of the ring finger. Injury to this nerve wouldn't primarily affect finger extension.
- **Option B:** The **ulnar nerve** mainly controls the intrinsic muscles of the hand (except those of the thenar eminence and lateral two lumbricals), flexion of the ring and little fingers, and sensation to the little finger and the ulnar half of the ring finger. It doesn't primarily affect wrist or finger extension.
- **Option D:** The **musculocutaneous nerve** primarily innervates the biceps brachii, brachialis, and coracobrachialis, which are involved in elbow flexion, not finger or wrist extension.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **radial nerve** wraps around the humerus in the radial groove, making it susceptible to injury with fractures of the humerus. A classic presentation of radial nerve injury is **wrist drop**, where the patient cannot extend the wrist.

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