**Core Concept**
Guyon's canal, also known as the ulnar canal, is a narrow anatomical structure that transmits the ulnar nerve and blood vessels in the wrist. It is a vital passage for the ulnar nerve, which plays a crucial role in hand function.

**Why the Correct Answer is Right**
The ulnar nerve passes through Guyon's canal, specifically the ulnar artery and vein, and the ulnar nerve itself. This canal is located on the ulnar (medial) aspect of the wrist, deep to the pisiform bone and the flexor carpi ulnaris tendon. The ulnar nerve is susceptible to compression within this canal, leading to symptoms of ulnar nerve entrapment, such as numbness and weakness in the hand.

**Why Each Wrong Option is Incorrect**
* **Option A:** The median nerve does not pass through Guyon's canal; it passes through the carpal tunnel.
* **Option B:** The radial artery passes through the first dorsal interosseous space, not Guyon's canal.
* **Option D:** The flexor pollicis longus tendon passes through the carpal tunnel, not Guyon's canal.

**Clinical Pearl / High-Yield Fact**
Guyon's canal syndrome, also known as ulnar nerve entrapment, is a common condition that presents with symptoms of ulnar nerve compression, including numbness and weakness in the hand, particularly affecting the little finger and ring finger.

**Correct Answer: C. The ulnar nerve.**