**Core Concept**
The flexor retinaculum of the wrist is a fibrous band that forms the carpal tunnel, which allows the passage of tendons and neurovascular structures to the hand. The structures that pass deep to the flexor retinaculum are primarily the median nerve, the tendon of the flexor pollicis longus, and the four unhar (ulnar) side thumb tendons.

**Why the Correct Answer is Right**
The median nerve passes deep to the flexor retinaculum, entering the carpal tunnel, where it is closely associated with the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and the flexor pollicis longus. The median nerve is susceptible to compression within the carpal tunnel, leading to carpal tunnel syndrome. The unhar side thumb tendons, including the tendons of the abductor pollicis longus and extensor pollicis brevis, pass superficial to the flexor retinaculum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so it cannot be evaluated.
**Option B:** This option is not provided, so it cannot be evaluated.
**Option C:** This option is not provided, so it cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
The median nerve is particularly susceptible to compression within the carpal tunnel due to its close association with the tendons and its relatively fixed position within the carpal tunnel.

**Correct Answer:** D