**Core Concept:** Falls on outstretched hand are a common mechanism of injury leading to fractures. The purpose of outstretched hand is to absorb impact during fall, which helps minimize injury to the upper limb.

**Why the Correct Answer is Right:** When a person falls on an outstretched hand, the hand acts as a cushion, reducing the force transmitted to the upper limb and minimizing the risk of fracture. The hand's structure with its bones, muscles, and ligaments helps distribute the impact evenly, reducing the pressure on a specific site.

In this case, the correct answer is **C.** Radius and ulna are long bones in the forearm that are often fractured due to the fall on outstretched hand. These bones are situated at the level of the wrist, making them more susceptible to fractures during such an event.

**Why Each Wrong Option is Incorrect:**

A. Colles' fracture (ulnar-sided injury) is the most common type of fracture resulting from a fall on an outstretched hand. This option is incorrect because it is not the correct answer.

B. Greenstick fracture (cervical spine) is an incomplete fracture that occurs in the pediatric population. Although falls on the hand can lead to cervical spine injuries, this option is incorrect because it focuses on a spinal fracture, not a forearm fracture.

D. Hip fracture (fall on the ground) is a fracture that occurs when a person falls on the ground, usually due to the impact of the fall. This option is incorrect because it discusses a fracture associated with a fall on the ground, not a fall on an outstretched hand.

**Clinical Pearl:** Understanding the mechanism of injury helps in diagnosing the type of fracture accurately. In a fall on an outstretched hand, the forearm bones are most commonly fractured due to the energy absorbed by the hand during injury. This knowledge allows clinicians to correctly diagnose and manage fractures resulting from falls on outstretched hands.