**Core Concept**
The flexor retinaculum, also known as the transverse carpal ligament, is a fibrous band that forms the roof of the carpal tunnel in the wrist. It provides a passage for the median nerve and the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and flexor pollicis longus muscles.

**Why the Correct Answer is Right**
The correct answer is the structure that does not pass through the carpal tunnel, which is formed by the flexor retinaculum. The median nerve, which supplies sensation to the thumb, index, middle finger, and half of the ring finger, passes through the carpal tunnel. The tendons of the flexor digitorum profundus, flexor digitorum superficialis, and flexor pollicis longus muscles also pass through the carpal tunnel to innervate the muscles of the hand. On the other hand, the ulnar nerve, which supplies sensation to the little finger and half of the ring finger, passes through Guyon's canal, which is a separate tunnel that does not involve the flexor retinaculum.

**Why Each Wrong Option is Incorrect**
**Option A:** Not provided.
**Option B:** Not provided.
**Option C:** Not provided.
**Option D:** Not provided.

**Clinical Pearl / High-Yield Fact**
The flexor retinaculum is a common site for carpal tunnel syndrome, a condition characterized by compression of the median nerve, leading to numbness, tingling, and weakness in the hand.

**Correct Answer:** D.