A 28-year-old obese woman presents with a 3-month history of progressive morning headaches, transient visual obscurations on bending over, and pulsatile tinnitus. Fundoscopy reveals blurred optic disc margins with obscuration of disc vessels and peripapillary flame hemorrhages. Notably, the findings marked **D** in the diagram are present. MRI brain and venography are normal. Lumbar puncture shows an opening pressure of 32 cm H₂O with normal CSF composition. Which of the following best explains why the clinical presentation in this case is papilledema rather than papillitis?