You are answering a USMLE-style medical question. Question: A 23-year-old woman comes to the physician because of a 2-month history of episodic headaches associated with dizziness, nausea, and vomiting. Over-the-counter pain medications have failed to reduce her symptoms. An MRI of the brain shows isolated dilation of the left lateral ventricle. This dilatation is most likely caused by blockade of which of the following structures? Options: A) Arachnoid villi B) Interventricular foramen C) Median aperture D) Lateral apertures Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.