A 38-year-old woman presents with a gradual onset headache and 5 days of drowsiness. The headache started in the occipital region and has been radiating to the apex. She recently returned from a holiday in Australia one week ago and has been experiencing poor appetite after a cough and cold since then. She has no medical history, takes only the oral contraceptive pill, and is a non-smoker who drinks minimally. On examination, she has a full range of neck movements and no photophobia. Her limb examination is unremarkable. A CT head without contrast reveals two small areas of subarachnoid blood in the right convexity. She denies any recent head trauma. What investigation is most likely to provide a conclusive diagnosis?