**Core Concept**
A cock-up splint is a type of orthotic device designed to provide support and immobilization to the wrist and hand in a specific position, typically with the wrist in extension and the fingers in flexion. This is achieved by adjusting the splint to hold the wrist in a cocked-up position.

**Why the Correct Answer is Right**
The primary function of a cock-up splint is to immobilize and support the wrist in extension, thereby reducing pain and inflammation associated with wrist injuries or conditions such as De Quervain's tenosynovitis. By maintaining the wrist in extension, the cock-up splint helps to reduce tension on the tendons and ligaments surrounding the wrist joint, promoting healing and preventing further injury.

**Why Each Wrong Option is Incorrect**
* **Option A:** A cock-up splint is not typically used for immobilizing the ankle or foot, as it is specifically designed for the wrist and hand.
* **Option B:** While a cock-up splint can provide support and stability to the wrist, it is not primarily used for treating conditions affecting the fingers or thumb.
* **Option C:** A cock-up splint is not used for immobilizing the elbow or forearm, as it is specifically designed to address wrist and hand injuries.

**Clinical Pearl / High-Yield Fact**
A cock-up splint is often used in conjunction with physical therapy and exercises to promote wrist mobility and strength after injury or surgery.

**Correct Answer:** C.