**Core Concept**
The carpal tunnel is a narrow anatomical structure in the wrist that contains the median nerve and tendons of the flexor muscles of the forearm. It is bounded by the carpal bones and the transverse carpal ligament. Understanding the contents of the carpal tunnel is crucial for diagnosing and managing conditions such as carpal tunnel syndrome.

**Why the Correct Answer is Right**
The carpal tunnel primarily contains the median nerve, which is responsible for innervating the muscles of the thenar eminence and providing sensation to the palmar surface of the thumb, index, middle finger, and the lateral half of the ring finger. The flexor digitorum superficialis (FDS) tendon, which is involved in flexing the fingers, also passes through the carpal tunnel. The flexor pollicis longus (FPL) tendon, responsible for thumb flexion, is another structure that traverses the carpal tunnel. In contrast, the flexor carpi ulnaris (FCU) tendon is not contained within the carpal tunnel; instead, it passes through a separate compartment in the wrist.

**Why Each Wrong Option is Incorrect**
**Option A:** The median nerve is indeed a content of the carpal tunnel, as explained above.
**Option B:** The FDS tendon is also a correct content of the carpal tunnel.
**Option C:** The FPL tendon is another structure that traverses the carpal tunnel.

**Clinical Pearl / High-Yield Fact**
When examining a patient with suspected carpal tunnel syndrome, it is essential to assess for median nerve compression symptoms, such as numbness, tingling, or weakness in the distribution of the median nerve. A positive Tinel's sign or Phalen's test can support the diagnosis.

**✓ Correct Answer: D. FCU tendon**