**Core Concept**
Gamekeeper's thumb is an injury to the ulnar collateral ligament of the thumb, also known as **skier's thumb**. This condition results from a sudden force that pulls the thumb away from the palm, leading to a partial or complete tear of the ligament. The **ulnar collateral ligament** plays a crucial role in thumb stability.

**Why the Correct Answer is Right**
The correct answer is related to the definition and common causes of Gamekeeper's thumb. This injury typically occurs due to a sudden, forceful abduction of the thumb, which can happen during sports like skiing or when someone falls onto their thumb. The **ulnar collateral ligament** is the primary stabilizer of the thumb, and its injury leads to instability and pain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe Gamekeeper's thumb.
**Option B:** This option is also incorrect as it does not relate to the ulnar collateral ligament injury.
**Option C:** Similarly, this option is incorrect as it does not define Gamekeeper's thumb.
**Option D:** This option might seem plausible but is incorrect because Gamekeeper's thumb is more specifically related to the ulnar collateral ligament injury.

**Clinical Pearl / High-Yield Fact**
Gamekeeper's thumb is a significant injury that can lead to chronic instability and pain if not properly treated. It is essential to recognize this condition early and provide appropriate management to prevent long-term complications.

**Correct Answer:** D. Injury to ulnar collateral ligament of the thumb.