**Core Concept**
Guyon's canal is an osseous tunnel in the distal part of the ulna, which transmits the ulnar nerve and blood vessels. It is located on the medial aspect of the wrist, and its contents play a crucial role in hand function and sensation.

**Why the Correct Answer is Right**
The ulnar nerve passes through Guyon's canal, which is a narrow osseous tunnel that provides a pathway for the nerve to reach the hand. The ulnar nerve is responsible for innervating the intrinsic muscles of the hand, except for the thenar muscles, and provides sensation to the little finger and half of the ring finger. The ulnar artery and vein also pass through Guyon's canal, supplying blood to the hand.

**Why Each Wrong Option is Incorrect**
**Option A:** The median nerve does not pass through Guyon's canal; instead, it passes through the carpal tunnel, which is a separate osseous tunnel in the wrist.
**Option B:** The ulnar artery and vein are correct contents of Guyon's canal, but the ulnar nerve is the primary structure that passes through this canal.
**Option C:** The radial artery does not pass through Guyon's canal; it passes through the first dorsal interosseous space.

**Clinical Pearl / High-Yield Fact**
Guyon's canal syndrome, also known as ulnar nerve compression at the wrist, can cause numbness, tingling, and weakness in the hand, especially in musicians who use a vibrating musical instrument.

**Correct Answer:** C. Ulnar nerve.