**Core Concept**
Mallet finger is a type of flexor tendon injury at the distal interphalangeal joint (DIPJ), typically caused by an avulsion fracture of the extensor tendon insertion on the distal phalanx. This injury occurs when the DIPJ is forced into flexion against resistance, often due to a sudden blow to the tip of the finger.

**Why the Correct Answer is Right**
The correct answer involves understanding the pathophysiology of mallet finger. When the DIPJ is forced into flexion against resistance, the extensor tendon is avulsed from its insertion on the distal phalanx, leading to an inability to extend the DIPJ. This results in a characteristic "dropped" appearance of the finger. The extensor tendon is responsible for extending the DIPJ, and its avulsion leads to a loss of function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mallet finger does not typically involve the proximal interphalangeal joint (PIPJ). The injury is specific to the DIPJ.
**Option B:** This option is incorrect because mallet finger is not caused by a direct blow to the back of the hand. The injury occurs at the DIPJ, not the wrist or hand.
**Option C:** This option is incorrect because mallet finger is not a type of arthritis. While it may cause inflammation and pain, the underlying cause is a flexor tendon injury, not a degenerative joint disease.

**Clinical Pearl / High-Yield Fact**
Mallet finger is a common injury in athletes, particularly those involved in sports that require hand-eye coordination, such as baseball and soccer. Early recognition and treatment are crucial to prevent long-term complications, including chronic pain and stiffness.

**Correct Answer: C. Mallet finger is a type of flexor tendon injury at the distal interphalangeal joint (DIPJ).**