**Core Concept**
The carpal tunnel is a narrow passageway in the wrist that contains the median nerve and tendons that control finger movement. Structures passing through the carpal tunnel are crucial for hand function and sensation. Understanding which structures pass through the carpal tunnel is essential for diagnosing and managing conditions such as carpal tunnel syndrome.

**Why the Correct Answer is Right**
The median nerve is a mixed nerve that carries sensory, motor, and sympathetic fibers. It passes through the carpal tunnel, which is formed by the carpal bones and the transverse carpal ligament. The median nerve supplies sensation to the palmar surface of the thumb, index, middle finger, and half of the ring finger. It also innervates the thenar muscles, which are responsible for thumb movement.

**Why Each Wrong Option is Incorrect**
* **Option A:** The ulnar nerve passes through Guyon's canal, not the carpal tunnel.
* **Option B:** The radial nerve passes through the radial tunnel, not the carpal tunnel.
* **Option C:** The flexor pollicis longus tendon does pass through the carpal tunnel, but it is not the correct answer in this context.

**Clinical Pearl / High-Yield Fact**
The carpal tunnel is a common site for compression neuropathy, particularly in individuals who perform repetitive wrist movements or have conditions such as diabetes or rheumatoid arthritis. Early recognition and treatment of carpal tunnel syndrome are essential to prevent permanent nerve damage.

**Correct Answer: D. Median nerve**