A previously healthy 27-year-old woman presented to the emergency department with 2 weeks of headache, photophobia, vomiting, and confusion. She denied fevers or chills. She was born in Guatemala and had immigrated to the US approximately 1 year ago, without subsequent international travel. She took no medications. On examination, she was afebrile (36.8 °C) and hemodynamically stable, with heart rate 70/min and blood pressure 95/53 mm Hg. She was somnolent and confused without focal neurologic deficits. Serum white blood cell count was elevated to 14.9 ×109/L (86% segmented neutrophils, 9% lymphocytes, 5% monocytes, 0% basophils/eosinophils). A computed tomography scan of her brain was notable for diffuse ventriculomegaly and sulcal effacement consistent with hydrocephalus and cerebral edema. Given her diminished level of consciousness, a ventriculostomy was placed emergently. Laboratory analysis of cerebrospinal fluid (CSF) obtained from the ventriculostomy was notable for 40 red blood cells/mm3; 3 nucleated cells/mm3; protein level, 11 mg/dL; and glucose level, 73 mg/dL. Magnetic resonance imaging (MRI) of the brain with and without contrast demonstrated ventriculomegaly of the lateral and third ventricles, with a 0.4 × 0.4 × 0.6–cm nonenhancing lesion obstructing the aqueduct of Sylvius (Figure 1A). No abnormal meningeal enhancement or parenchymal lesion was identified. On hospital day 5, in the midst of an ongoing infectious diseases workup, the measured intracranial pressure and rate of CSF drainage from the ventriculostomy suddenly decreased. Repeat MRI demonstrated resolved hydrocephalus, a patent aqueduct of Sylvius, and the aforementioned nonenhancing lesion now located in the fourth ventricle (Figure 1B).