**Multiple Choice Explanation**

### Core Concept

Non-union is a condition where the fractured bone fails to heal or unite with the adjacent bone tissue, leading to a prolonged or permanent defect. This complication often arises due to inadequate blood supply, infection, or mechanical instability at the fracture site.

### Why the Correct Answer is Right

The most common cause of non-union is **mechanical instability**, which can be due to excessive movement or stress at the fracture site. This disrupts the normal bone healing process, characterized by the coordinated activity of osteoblasts (bone-forming cells), osteoclasts (bone-resorbing cells), and growth factors. Inadequate immobilization or the use of inadequate internal fixation devices can contribute to mechanical instability, making non-union more likely.

### Why Each Wrong Option is Incorrect

* **Option A:** (no option provided, skipping to the next)
* **Option B:** While infection is a known complication of fractures, it is not the most common cause of non-union. Infection can lead to non-union, but it is often a secondary complication of inadequate wound care or delayed treatment.
* **Option C:** (no option provided, skipping to the next)
* **Option D:** (no option provided, skipping to the next)

### Clinical Pearl / High-Yield Fact

A key factor in preventing non-union is ensuring **adequate immobilization** of the fracture site. This can be achieved through the use of internal fixation devices, such as plates, screws, or rods, or external fixation devices, such as casts or braces.

### Correct Answer:
(No answer provided, please fill in the correct option)