**Core Concept**
A fluid-filled cavity at the center of the spinal cord is a characteristic feature of a specific pathological condition. This condition is related to the development and structure of the central nervous system, particularly the spinal cord. The fluid-filled cavity is an abnormality that can result from various factors, including congenital anomalies, infections, or traumatic injuries.

**Why the Correct Answer is Right**
The correct condition is associated with the formation of a central canal within the spinal cord, which is a normal feature in the embryonic development of the central nervous system. However, in certain pathological conditions, this central canal can become dilated or enlarged, forming a fluid-filled cavity. This cavity can be filled with cerebrospinal fluid (CSF), which is a clear, colorless fluid that surrounds the brain and spinal cord. The central canal is lined by ependymal cells, which are specialized cells that produce and maintain the CSF.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically refer to a fluid-filled cavity at the center of the spinal cord. While syringomyelia is a condition that involves the spinal cord, it typically involves the formation of a fluid-filled cavity within the spinal cord parenchyma, rather than the central canal.

**Option B:** This option is incorrect because it is not a specific or well-defined condition related to a fluid-filled cavity at the center of the spinal cord. While there are various conditions that can affect the spinal cord, this option does not accurately describe a known pathological condition.

**Option D:** This option is incorrect because it does not specifically refer to a fluid-filled cavity at the center of the spinal cord. While arachnoid cysts can occur in the spinal canal, they are typically located in the subarachnoid space, rather than within the spinal cord itself.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that a fluid-filled cavity at the center of the spinal cord can be associated with symptoms such as pain, weakness, and sensory disturbances. These symptoms can be caused by the compression or damage to the spinal cord parenchyma, and can be exacerbated by the presence of a fluid-filled cavity.

**Correct Answer: C. Syringomyelia**