I have a friend suffering from dizziness (but not vertigo), periodic headaches (doesn't exactly fit the description of a migraine), and slightly elevated blood pressure when going from lying down to sitting or standing up. Her MRI workup (including DWI, T2*, T2 FLAIR, T1, and GE) was within normal limits, and her ophthalmologist did not detect any vision problems, nor changes in the optic disc. The neurologist would like to do a spinal tap next, but she has no infection (no fever or other infection-related s/sx), and no visible tumor or anatomical distortions from the MRI. What is the neurologist looking for in the tap?