**Core Concept**
Bennett's fracture is a specific type of fracture-dislocation that involves the base of one of the metacarpal bones in the hand. This injury is characterized by a fracture of the metacarpal neck with a dislocation of the carpometacarpal joint.

**Why the Correct Answer is Right**
The correct answer is the fifth metacarpal because Bennett's fracture specifically involves the base of the fifth metacarpal bone. This type of fracture occurs when the metacarpal neck is compressed and the carpometacarpal joint is dislocated. The mechanism of injury often involves forced abduction of the fifth metacarpal, leading to a fracture of the neck and a dislocation of the carpometacarpal joint.

**Why Each Wrong Option is Incorrect**
**Option A:** The first metacarpal is involved in a different type of fracture-dislocation called a "scaphoid fracture" or "scaphoid waist fracture", not Bennett's fracture.
**Option B:** The second metacarpal is not typically associated with Bennett's fracture.
**Option C:** The third metacarpal is not involved in Bennett's fracture.
**Option D:** The fourth metacarpal is also not typically associated with Bennett's fracture.

**Clinical Pearl / High-Yield Fact**
Bennett's fracture is a common injury in young adults, particularly those involved in sports or activities that require gripping or grasping. It is essential to diagnose and treat this type of fracture promptly to prevent long-term complications and ensure proper healing.

**Correct Answer:** D. Fifth metacarpal.