A 32-year-old woman on combined oral contraceptive pills (35 µg ethinyl estradiol + norethisterone) for 2 years presents with sudden-onset severe unilateral headache with photophobia and neck stiffness. She denies any recent trauma or fever. Blood pressure is 140/90 mmHg. On neurological examination, she has positive Kernig's and Brudzinski's signs. What is the most appropriate immediate next step in management?