A 33-year-old woman complains of generalized, throbbing headache that is worse in the morning and with coughing. She occasionally feels dizzy and nauseated. Examination is significant only for obesity and bilateral papilledema. CT scan of the head is nominal. At lumbar puncture the opening pressure is 220 mm H2O; CSF is clear, with protein of 12 mg/100 mL (normal is 15 to 45), glucose of 68 mg/100 mL (normal is 45 to 80), an d no cells are seen. The most likely diagnosis is: