aRbor vitae... part of the cerebellum, notably the paleocerebellum.
Your diagnosis would depend on the specific signs and symptoms, labs, and imaging results. I was aiming at medulloblastoma, but the bottom line is that the COMLEX will give you classic clinical buzzwords and expect you to know the diagnosis, the pathological cell type, or biochemical deficit, or the anatomical structure involved.
Not EVERY question is like this but it IS more common. Good luck.