**Question:** Mallet finger is a flexion deformity of the distal phalanx, which occurs due to:

**Core Concept:** Mallet finger is a clinical condition characterized by abnormal flexion deformity of the distal phalanx, resulting from a rupture or sprain of the extensor tendon. The extensor tendon is composed of the long digital extensor tendon, which runs along the dorsal aspect of the distal phalanx, and is responsible for extending the finger.

**Why the Correct Answer is Right:** The correct answer (C) refers to the rupture or sprain of the long digital extensor tendon, which leads to the flexion deformity of the distal phalanx in mallet finger.

**Why Each Wrong Option is Incorrect:**

A. Infection (D) is not related to the extensor tendon and flexion deformity of the finger.
B. Trauma (B) can cause a rupture or sprain of the extensor tendon, leading to mallet finger. However, options C and D are more specific explanations for the condition.

**Clinical Pearl / High-Yield Fact:**

In mallet finger, the ruptured or sprained extensor tendon can be visualized as a "double-bunched" appearance at the site of injury, as the tendon moves in two layers. This characteristic feature helps in making the correct diagnosis and distinguishes it from other similar conditions.

**Correct Answer:** C. Rupture or sprain of the long digital extensor tendon.