What condition is characterized by excess fluid in the spinal cord?

Syringomyelia is characterized by the presence of a fluid-filled cyst or cavity (referred to as a syrinx) that forms within the spinal cord itself. This condition often leads to a range of neurological symptoms, which can include pain, weakness, stiffness, or sensory abnormalities due to the pressure and disruption of the normal structure and function of the spinal cord.

In contrast, hydrocephalus involves the accumulation of cerebrospinal fluid in the brain's ventricles, leading to increased intracranial pressure rather than affecting the spinal cord directly. Arachnoid cysts, also distinct from syringomyelia, are fluid-filled sacs that can occur within the arachnoid space in the brain or along the spinal cord but do not specifically indicate excess fluid within the spinal cord tissue itself. Multiple sclerosis is a demyelinating disease of the central nervous system that can cause neurological impairment but is not primarily characterized by fluid accumulation in the spinal cord.

Understanding these distinctions highlights why syringomyelia is the correct answer, as it specifically describes the condition where excess fluid resides within the spinal cord.