**Core Concept**
The anatomical snuffbox is a triangular deepening on the radial, dorsal aspect of the hand—at the level of the carpal bones, specifically the scaphoid and trapezium bones. It is bounded by tendons. The **extensor pollicis longus**, **extensor pollicis brevis**, and **abductor pollicis longus** tendons form its borders.

**Why the Correct Answer is Right**
The medial boundary is formed by the **extensor pollicis longus** tendon. This tendon plays a crucial role in thumb extension. The anatomical snuffbox is significant for various clinical examinations, including assessing the integrity of the scaphoid bone.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it does not form the medial boundary.
**Option B:** is incorrect as it is not related to the anatomical snuffbox's medial border.
**Option C:** and **Option D:** are not the correct answers as they do not accurately represent the medial boundary of the anatomical snuffbox.

**Clinical Pearl / High-Yield Fact**
The anatomical snuffbox is a key area for clinical examination, especially for fractures of the scaphoid bone, as tenderness in this area can indicate such injuries.

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