**Question:** Carpal tunnel syndrome is due to involvement of which nerve: March 2009

**Core Concept:**
Carpal tunnel syndrome is a common clinical entity characterized by symptoms of median nerve compression at the wrist level. The median nerve is a mixed nerve, containing sensory, motor, and autonomic fibers. It arises from the cervical spinal cord (C8-T1) and provides innervation to the skin, muscles, and organs of the hand.

**Why the Correct Answer is Right:**
The correct answer is D - the ulnar nerve.

In carpal tunnel syndrome, compression occurs at the wrist level and involves the median nerve. The ulnar nerve, however, is usually spared due to its more distal path and separate entry into the forearm. While the median nerve innervates the thumb, index, middle, and half of the ring finger, the ulnar nerve innervates the small finger, half of the ring finger, and the medial two-thirds of the index finger and the thumb's ulnar aspect.

**Why Each Wrong Option is Incorrect:**
A - The radial nerve is responsible for innervating the muscles of the radial side of the hand and is not involved in carpal tunnel syndrome.
B - The posterior interosseous nerve is a spinal branch of the radial nerve and does not contribute to carpal tunnel syndrome.
C - The posterior tibial nerve is a spinal branch of the lumbosacral plexus and is not involved in carpal tunnel syndrome.

**Core Concept:**
Carpal tunnel syndrome is a clinical entity characterized by symptoms due to compression of the median nerve at the wrist level.

**Why the Correct Answer is Right:**
The correct answer, D - the ulnar nerve, is right because the median nerve is the primary target in carpal tunnel syndrome, as it passes through the carpal tunnel, a fibro-osseous constriction of the wrist bones. Compression of the median nerve causes symptoms such as pain, numbness, and weakness in the distribution of the median nerve.

**Why Each Wrong Option is Incorrect:**
A - The radial nerve is not involved in carpal tunnel syndrome because it passes through the Guyon's canal, not the carpal tunnel.
B - The posterior interosseous nerve is a spinal branch of the radial nerve and does not contribute to carpal tunnel syndrome.
C - The posterior tibial nerve is a spinal branch of the lumbosacral plexus and does not participate in carpal tunnel syndrome.

**Core Concept:**
Carpal tunnel syndrome is a clinical entity characterized by symptoms due to compression of the median nerve at the wrist level.

**Why the Correct Answer is Right:**
The correct answer, D - the ulnar nerve, is right because the median nerve is the primary target in carpal tunnel syndrome, which is caused by compression of the median nerve within the carpal tunnel of the wrist.

**Why Each Wrong Option is Incorrect:**
A - The radial nerve is not involved in carpal tunnel syndrome because it passes through Guyon's canal, not the carpal tunnel.
B - The posterior inter