A 28-year-old female presents to the Emergency department with complaints of headaches and diplopia. She reports experiencing a generalised and throbbing headache with associated nausea since the morning. Over the last two hours, she has noticed pain and double vision when looking to the right, and her left eyelid has started to droop. She denies any changes in facial sensation or weakness in her arms or legs. Her medical history includes migraines, for which she occasionally takes Imigran. She is a smoker and drinks alcohol regularly.
On examination, she is alert and oriented with mild nuchal rigidity. Fundoscopy is normal, but she has photophobia. Cranial nerve examination reveals a left oculomotor nerve palsy with left-sided ptosis and mydriasis. There is no evidence of chemosis or proptosis, and the rest of the cranial nerves and visual fields appear normal. Peripheral nervous system examination is unremarkable.
Laboratory investigations show a slightly low serum potassium level and a mildly elevated C-reactive protein level. Given the clinical presentation, which investigation is most likely to lead to a definitive diagnosis?