**Core Concept**
The anatomical snuff box is a triangular deepening on the radial, dorsal aspect of the hand. Its boundaries are formed by the tendons of the muscles of the forearm. Understanding the boundaries of the anatomical snuff box is crucial for diagnosing various conditions affecting the hand and wrist.

**Why the Correct Answer is Right**
The medial boundary of the anatomical snuff box is formed by the tendon of the flexor carpi radialis muscle. This tendon serves as a landmark for the surgeon and clinician to identify the location of the radial artery, which is commonly palpated at the level of the anatomical snuff box. The tendon of the flexor carpi radialis muscle is a key structure that helps in identifying the medial limit of the anatomical snuff box.

**Why Each Wrong Option is Incorrect**
**Option A:** The tendon of the extensor pollicis longus muscle actually forms the lateral boundary of the anatomical snuff box, not the medial boundary.
**Option B:** The radial artery is located within the anatomical snuff box, but it does not form the medial boundary of the snuff box.
**Option C:** The tendon of the brachioradialis muscle is not a boundary of the anatomical snuff box.

**Clinical Pearl / High-Yield Fact**
The anatomical snuff box is a clinically important landmark for diagnosing conditions such as De Quervain's tenosynovitis, which affects the tendons of the abductor pollicis longus and extensor pollicis brevis muscles.

**Correct Answer: C. The tendon of the flexor carpi radialis muscle.**