**Core Concept**
Mallet finger is an injury to the extensor tendon of the distal phalanx, typically caused by a sudden forceful flexion of the fingertip. This results in an inability to actively extend the distal interphalangeal joint.

**Why the Correct Answer is Right**
The mechanism of mallet finger involves a sudden, forceful flexion of the fingertip, which ruptures the extensor tendon of the distal phalanx. This injury is often seen in sports, particularly those involving racquet sports or football, where the fingertip is subjected to a sudden, forceful impact. The extensor tendon is responsible for extending the distal interphalangeal joint, and its rupture leads to an inability to actively extend the joint.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the mechanism of mallet finger involves a direct blow to the fingertip, not a flexion of the wrist or forearm.
* **Option B:** This option is incorrect because the injury to the flexor tendon would result in an inability to flex the distal interphalangeal joint, not an inability to extend it.
* **Option D:** This option is incorrect because a fracture of the distal phalanx would result in pain, swelling, and difficulty moving the joint, but it would not specifically cause an inability to extend the joint.

**Clinical Pearl / High-Yield Fact**
To diagnose mallet finger, look for an inability to actively extend the distal interphalangeal joint, particularly in the context of a recent trauma or sports injury. A simple test to confirm the diagnosis is to ask the patient to actively extend the joint against resistance.

**Correct Answer:** C.