**Core Concept**
A tube cast is a type of orthotic device used to immobilize and stabilize a fractured limb, providing support and protection to the affected area during the healing process. This method is commonly employed in cases of fractures that require immobilization, particularly in the upper limb.

**Why the Correct Answer is Right**
The application of a tube cast is typically reserved for fractures that are relatively stable and do not require surgical intervention. The cast provides a rigid framework that helps maintain the alignment of the bones, thereby facilitating proper healing. In the context of the upper limb, a tube cast may be used for fractures of the humerus, radius, or ulna, among other sites.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a splint is a more flexible and removable device used to support and stabilize a limb, whereas a tube cast is a more rigid and non-removable device.
**Option B:** This option is incorrect because a walking cast is a type of cast used to support the foot and ankle, particularly after fractures or surgical procedures.
**Option C:** This option is incorrect because a back slab is a type of cast used to immobilize a fractured limb in the early stages, but it is typically used for lower limb fractures.

**Clinical Pearl / High-Yield Fact**
When applying a tube cast, it is essential to ensure proper alignment and immobilization of the affected limb to prevent complications such as malunion, nonunion, or nerve damage.

**Correct Answer: D.**