TeesNeuro.org: A 50 year old man presents to A&E with a sudden onset headache which came on 2 hours ago. He has associated nausea and neck stiffness. Examination is unremarkable. He is haemodynamically stable and apyrexic, but sensitive to the light. He has a CT brain which is normal. What would you do next?

A 50 year old man presents to A&E with a sudden onset headache which came on 2 hours ago. He has associated nausea and neck stiffness. Examination is unremarkable. He is apyrexic but sensitive to the light. CT head is NAD however on lumbar puncture, CSF is positive for xanthochromia. What is the most likely underlying cause of this current event?