This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Fluid-filled cavity at the center of the spinal cord is seen in: 
Select and explain. 
A. Syringomyelia, B. Brown Sequard syndrome, C. Tabes dorsalis, D. Subacute combined degeneration (SACD) of cord
A: Syringomyelia
Answer: A: Syringomyelia 
Explanation: In syringomyelia, a cystic cavity forms within the spinal cord.
This cyst, called a syrinx, can expand and elongate over time, destroying the spinal cord.
Typically seen in cervical areas, there is dissociative anaesthesia (pain and  temperature ie spinothalamic sensations are lost, while posterior coloumn sensation ie proprioception and vibration are preserved) in a cape shaped distribution.
Brown sequard syndrome is seen in hemisection of the cord .
Tabes dorsalis is syphilitic damage to cord.
SACD occurs due to Vitamin B12 deficiency.