## **Core Concept**
Carpal tunnel syndrome (CTS) is a medical condition that arises from compression of a vital structure in the wrist. The condition is characterized by numbness, tingling, and pain in the hand, particularly in the distribution of the thumb, index, and middle fingers. The underlying cause is the compression of a specific nerve as it passes through a confined space in the wrist.

## **Why the Correct Answer is Right**
The correct answer, **median nerve**, is the structure compressed in carpal tunnel syndrome. The median nerve, along with tendons, passes through the carpal tunnel, a narrow passageway on the palmar side of the wrist. Compression of the median nerve within this tunnel leads to the symptoms associated with CTS. The median nerve is responsible for innervating muscles that control thumb opposition and for providing sensation to the palmar surface of the thumb, index, middle finger, and the radial half of the ring finger.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The ulnar nerve is not compressed in carpal tunnel syndrome; instead, it is often associated with cubital tunnel syndrome or ulnar neuropathy at the elbow.
- **Option B:** The radial nerve is primarily responsible for wrist and finger extension and is not compressed in carpal tunnel syndrome; its compression leads to radial neuropathy or "wrist drop."
- **Option D:** The musculocutaneous nerve primarily supplies the biceps brachii, brachialis, and coracobrachialis muscles and does not pass through the carpal tunnel.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for carpal tunnel syndrome is the use of Tinel's sign for diagnosis. Tinel's sign is elicited by lightly tapping over the median nerve in the wrist; a tingling sensation in the distribution of the median nerve indicates a positive test. This simple bedside test can help confirm the diagnosis of CTS.

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