## **Core Concept**
Mallet finger, also known as baseball finger, is a condition characterized by the inability to extend the distal interphalangeal (DIP) joint of the finger. It typically occurs due to a direct blow to the tip of the finger, causing forced flexion of the DIP joint. The underlying injury involves the extensor tendon.

## **Why the Correct Answer is Right**
The correct answer involves the injury to the extensor tendon at its insertion on the **distal phalanx**. The extensor tendon plays a crucial role in extending the DIP joint. When a forceful object strikes the tip of the finger, it causes sudden flexion of the DIP joint, leading to a rupture or avulsion fracture of the extensor tendon at its insertion site on the distal phalanx. This injury disrupts the extensor mechanism, preventing active extension of the DIP joint.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **proximal phalanx** is not the site of extensor tendon insertion related to DIP joint extension. The extensor tendon does insert here but for the proximal interphalangeal (PIP) joint.
- **Option B:** The **middle phalanx** is also not directly related to the insertion of the extensor tendon for DIP joint extension.
- **Option C:** Not specified but any other location not directly related to the DIP joint extensor mechanism would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that mallet finger can be treated conservatively with splinting to allow the tendon to heal, but it requires prompt attention to prevent long-term disability. Untreated or undertreated cases can lead to chronic problems, including swan neck deformity.

## **Correct Answer:** D. distal phalanx.