**Core Concept:** The third extensor compartment of the wrist is a subdivision of the lateral compartment, where tendons of extensor muscles pass through. It is responsible for extending the digits of the hand.

**Why the Correct Answer is Right:** The correct answer is "D." The third extensor compartment contains the tendon of the extensor pollicis longus muscle. This muscle is crucial for extending the thumb and index finger. Its function is to stabilize the wrist in a neutral position and prevent excessive pronation of the hand.

**Why Each Wrong Option is Incorrect:**

A. The first extensor compartment contains the tendons of the flexor pollicis longus and flexor digitorum profundus muscles. These muscles are involved in flexing the digits of the hand, not extending them.

B. The second extensor compartment contains the tendons of the flexor digitorum superficialis and flexor digitorum profundus muscles, which are responsible for flexing the digits of the hand.

C. The fourth extensor compartment contains the tendons of the lumbricals and interossei muscles, which are involved in opposing the fingers and moving them between the web spaces, not extending them.

**Clinical Pearl:** Understanding the anatomy of the extensor tendons is essential for preventing complications during surgical procedures involving the wrist and hand. Misplacing tendons can lead to impaired hand function, which is why it's crucial for medical professionals to be familiar with these structures.

**Correct Answer:** D. Extensor pollicis longus