A 35-year-old man with chronic headaches and papilloedema on fundoscopy is found to have elevated intracranial pressure (ICP) on lumbar puncture (opening pressure 35 cm H₂O). Visual acuity is preserved, there is no eye pain, and the optic disc margins are blurred bilaterally with venous engorgement. No demyelinating lesions are seen on brain MRI. What is the drug of choice for acute management of this condition?