**Core Concept**
The anatomical snuff box is a triangular deepening on the radial, lateral aspect of the forearm and wrist. It is bounded by three tendons of the thumb: the Abductor Pollicis Longus (APL), Extensor Pollicis Longus (EPL), and Extensor Pollicis Brevis (EPB). These tendons form the boundaries of the anatomical snuff box, which is an important landmark in wrist anatomy.

**Why the Correct Answer is Right**
The Extensor Carpi Ulnaris (ECU) tendon does not form a boundary of the anatomical snuff box. Instead, it lies on the ulnar side of the wrist and forearm, separate from the snuff box. The ECU tendon is involved in wrist extension and ulnar deviation, whereas the APL, EPL, and EPB tendons are involved in thumb extension. The anatomical snuff box is a critical area for surgeons and medical professionals to identify when performing procedures on the wrist and forearm.

**Why Each Wrong Option is Incorrect**
**Option A:** APL is one of the boundaries of the anatomical snuff box. It is a tendon that helps to abduct the thumb.
**Option B:** EPL is another tendon that forms the boundary of the anatomical snuff box. It is involved in thumb extension.
**Option C:** EPB is also a tendon that helps to form the boundary of the anatomical snuff box. It is involved in thumb extension.

**Clinical Pearl / High-Yield Fact**
The anatomical snuff box is a critical landmark in wrist anatomy, and identifying its boundaries is essential for surgeons and medical professionals. The APL, EPL, and EPB tendons can be palpated to identify the snuff box, which can be useful in diagnosing conditions such as De Quervain's tenosynovitis.

**✓ Correct Answer: D. ECU. The Extensor Carpi Ulnaris tendon does not form a boundary of the anatomical snuff box.**