**Core Concept**
Carpal tunnel syndrome is a condition characterized by the compression of a specific nerve within the wrist, leading to symptoms such as numbness, tingling, and weakness in the hand. This compression occurs due to the anatomical structure of the wrist, where the median nerve is susceptible to compression by the carpal bones and the flexor retinaculum.

**Why the Correct Answer is Right**
The median nerve is compressed in carpal tunnel syndrome due to its anatomical location within the wrist. The nerve passes through the carpal tunnel, a narrow space bounded by the carpal bones and the flexor retinaculum. When the median nerve is compressed, it can lead to symptoms such as numbness and tingling in the thumb, index finger, middle finger, and part of the ring finger. This compression can occur due to various factors, including repetitive hand movements, fluid retention, and anatomical abnormalities.

**Why Each Wrong Option is Incorrect**
**Option B:** The ulnar nerve is compressed in conditions such as cubital tunnel syndrome, not carpal tunnel syndrome. The ulnar nerve passes through the cubital tunnel at the elbow, not the carpal tunnel at the wrist.
**Option C:** The radial nerve is responsible for wrist extension and is not typically compressed in carpal tunnel syndrome. Compression of the radial nerve can lead to conditions such as radial tunnel syndrome.
**Option D:** The axillary nerve is not related to carpal tunnel syndrome, as it primarily innervates the deltoid muscle and the skin over the lateral aspect of the shoulder.

**Clinical Pearl / High-Yield Fact**
Carpal tunnel syndrome can be diagnosed using clinical examination, including the Tinel's sign and Phalen's test. Early recognition and treatment of carpal tunnel syndrome are crucial to prevent permanent nerve damage and improve symptoms.

**✓ Correct Answer: A. Median nerve**