In subarachnoid hemorrhage, which CSF finding is most characteristic?

• A. Normal opening pressure with clear CSF.
• B. Elevated opening pressure with a high protein level.
• C. RBCs present in CSF with possible xanthochromia.
• D. No red blood cells in CSF; glucose normal.

Answer: (C)

The key idea is that bleeding into the subarachnoid space leaves blood in the CSF. After rupture, red blood cells enter the CSF, so the CSF is bloody on analysis. As those RBCs break down, bilirubin forms and can give the CSF a yellowing, called xanthochromia. This combination—red blood cells in the CSF with the possible development of xanthochromia—is the most characteristic finding in subarachnoid hemorrhage. Timing matters: early on you may see RBCs without xanthochromia, since bilirubin takes a few hours to appear. Other patterns, like clear CSF with normal opening pressure, or absence of red cells, don’t reflect SAH, while elevated protein alone isn’t as specific.