## Core Concept
The anatomical snuffbox is a triangular deepening on the radial, dorsal aspect of the wrist. It is bounded by tendons of the **extensor pollicis longus**, **extensor pollicis brevis**, and **abductor pollicis longus** muscles. Understanding the anatomy of the wrist and the structures forming the anatomical snuffbox is crucial for clinical examination and diagnosis.

## Why the Correct Answer is Right
The medial boundary of the anatomical snuffbox is formed by the tendon of the **extensor pollicis longus** muscle. This muscle is responsible for the extension of the thumb, particularly the distal phalanx. The tendon runs in a groove on the ulnar (medial) side of the anatomical snuffbox, making it the correct medial boundary.

## Why Each Wrong Option is Incorrect
* **Option A:** Incorrect because the lateral boundary of the anatomical snuffbox is formed by the tendons of the **abductor pollicis longus** and **extensor pollicis brevis** muscles, not the medial boundary.
* **Option B:** Incorrect because, although it might seem related, the specific muscle or structure forming the medial boundary is the **extensor pollicis longus**, not another muscle or structure not specified.
* **Option D:** Incorrect because the **flexor pollicis longus** muscle is not a boundary of the anatomical snuffbox; it is involved in thumb flexion and is located more anteriorly.

## Clinical Pearl / High-Yield Fact
A key clinical correlation is that tenderness in the anatomical snuffbox can indicate a **scaphoid fracture**, which is a common wrist injury. The scaphoid bone is located near this region, and fractures here can be challenging to diagnose and manage.

## Correct Answer Line
**Correct Answer: C. extensor pollicis longus.**