**Core Concept**
The simple bone cyst, also known as a unicameral bone cyst, is a type of bone lesion that primarily affects the long bones of children and adolescents. It is characterized by the presence of a fluid-filled cavity within the bone, which can lead to pain, swelling, and potentially, bone fragility.

**Why the Correct Answer is Right**
The simple bone cyst typically occurs in the metaphyseal region of long bones, where bone growth is most active. The cyst is thought to arise from a failure of bone growth and remodeling, leading to the accumulation of fluid within the bone. The metaphysis is a region of high bone turnover, making it a susceptible site for the development of simple bone cysts.

**Why Each Wrong Option is Incorrect**
* **Option A:** The epiphyseal region is not a common site for simple bone cysts. While some cysts may occur here, the metaphysis is the more typical location.
* **Option B:** The diaphysis, or shaft of the bone, is not a common site for simple bone cysts. Cysts in this region are often referred to as "diaphyseal pseudoarthroses."
* **Option C:** The growth plate, or physis, is not a common site for simple bone cysts. While some cysts may occur here, the metaphysis is the more typical location.

**Clinical Pearl / High-Yield Fact**
The simple bone cyst is a relatively common bone lesion in children and adolescents, and its diagnosis is often made based on radiographic findings. It is essential to distinguish simple bone cysts from other bone lesions, such as aneurysmal bone cysts or unicameral bone cysts with secondary aneurysmal bone changes.

**Correct Answer:** D. Metaphysis of long bones.