A 25-year-old female presents with a persistent headache for the past eight weeks, which has worsened in the last week. She reports a constant frontal headache that is not relieved by paracetamol. The headache is present when she wakes up in the morning and persists throughout the day. Additionally, she has gained over 7 kg in weight in the last six months. On examination, she appears tearful and has a BMI of 32 kg/m2. However, there is no nuchal rigidity, and neurological examination is normal except for bilateral optic disc swelling on fundus examination. Her blood pressure is 122/88 mmHg, and her temperature is 37°C. What is the most appropriate investigation to make a diagnosis?