**Core Concept:** Treatment of forearm fractures involves reducing the fracture and immobilizing the affected limb to allow for healing. In adults, the two most common types of forearm fractures are the shaft fractures (diaphyseal) and the distal radio-ulnar joint (DRUJ) fractures. The treatment depends on the fracture type and the degree of displacement.

**Why the Correct Answer is Right:**
For a shaft fracture of the radius and ulna, the treatment of choice is **C cast immobilization**. This type of cast is specifically designed to immobilize the forearm in a neutral position to prevent further displacement of the fracture fragments. This type of cast is also known as a "short arm cast" or "cast paddle," and it is placed from the wrist to the elbow joint. The cast is applied with the forearm in neutral position, ensuring that there is no rotation, flexion, or extension of the forearm. The cast is typically held in place with adhesive tape and dressings to prevent any dislocation during the healing process.

**Why Each Wrong Option is Incorrect:**
A. A long arm cast (universal cast) is not suitable for shaft fractures of the radius and ulna as it doesn't maintain the forearm in a neutral position, which can lead to malunion or nonunion of the fracture.
B. Plate fixation is not the first choice for treating shaft fractures in adults, as it requires an operating room procedure, general anesthesia, and a longer recovery period.
D. Surgical reduction and internal fixation is typically reserved for displaced or unstable fractures, and not for simple shaft fractures without significant displacement.

**Clinical Pearl:**
The treatment of a shaft fracture of the radius and ulna in adults is crucial to ensure proper healing and avoid complications such as malunion, nonunion, or stiffness of the forearm. It is essential to maintain the forearm in a neutral position during the healing process, which is achieved through the use of a **C cast immobilization**. This type of cast ensures the stability of the fracture site and allows for a smooth healing process without causing further trauma to the fracture site.