## **Core Concept**
Carpal tunnel syndrome (CTS) is a medical condition that arises due to the compression of a specific nerve as it travels through the wrist. The wrist contains a band of fibrous tissue known as the transverse carpal ligament, which, along with the carpal bones, forms the carpal tunnel. Through this tunnel, the **median nerve**, along with the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and flexor pollicis longus muscles, passes.

## **Why the Correct Answer is Right**
The correct answer is the median nerve. The **median nerve** is compressed in carpal tunnel syndrome. This compression occurs due to the narrowing of the carpal tunnel, which can be caused by various factors such as swelling of the synovial sheaths around the tendons, tenosynovitis, or direct pressure on the wrist. The median nerve provides sensation to the palmar surface of the thumb, index, middle finger, and the lateral half of the ring finger. It also supplies the thenar muscles (thumb muscles), enabling thumb movement. Compression of this nerve leads to symptoms such as pain, numbness, and weakness in the hand.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The ulnar nerve is not compressed in carpal tunnel syndrome. It is involved in a different condition known as cubital tunnel syndrome or ulnar nerve entrapment, where it is compressed at the elbow.
- **Option B:** The radial nerve is not associated with carpal tunnel syndrome. It can be compressed in conditions like radial tunnel syndrome or wrist drop.
- **Option D:** The posterior interosseous nerve, a branch of the radial nerve, is also not related to carpal tunnel syndrome.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature of carpal tunnel syndrome is the presence of **Tinel's sign**, which is a tingling sensation in the distribution of the median nerve when the nerve is lightly tapped over the wrist. This is a useful diagnostic clue. Additionally, patients often report worsening of symptoms at night, which can disrupt sleep.

## **Correct Answer:** . Median nerve.