**Core Concept**
Carpal tunnel syndrome (CTS) is a condition characterized by compression of the median nerve as it passes through the carpal tunnel in the wrist. This compression leads to symptoms such as numbness, tingling, and weakness in the hand, particularly in the thumb, index, and middle fingers.

**Why the Correct Answer is Right**
The median nerve is compressed within the carpal tunnel, which is a narrow passageway formed by bones and ligaments in the wrist. The compression of the median nerve can be caused by various factors, including swelling of the tendons, fluid retention, or anatomical abnormalities. The compression disrupts the normal function of the median nerve, leading to the characteristic symptoms of CTS.

**Why Each Wrong Option is Incorrect**
**Option A:** The ulnar nerve is not compressed in carpal tunnel syndrome. The ulnar nerve runs through a separate tunnel in the wrist, known as Guyon's canal.
**Option B:** The radial nerve is not involved in carpal tunnel syndrome. The radial nerve runs along the back of the arm and forearm, and its compression leads to symptoms such as wrist drop.
**Option C:** The brachial plexus is a complex network of nerves that arise from the spinal cord and supply the arm. It is not compressed in carpal tunnel syndrome.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of carpal tunnel syndrome is nocturnal paresthesia, which refers to the numbness or tingling sensation that occurs in the hand during sleep. This symptom is often exacerbated by flexion of the wrist.

**Correct Answer: C. Median nerve. Median nerve compression within the carpal tunnel leads to symptoms of carpal tunnel syndrome.**