**Core Concept**
The question is testing the understanding of cerebrospinal fluid (CSF) dynamics and the structures involved in its circulation. The correct answer requires knowledge of the pathways through which CSF flows and the implications of obstruction in these pathways.

**Why the Correct Answer is Right**
CSF is produced in the choroid plexus of the ventricles and flows through the ventricular system into the subarachnoid space. The arachnoid villi, also known as the arachnoid granulations, are key structures involved in the reabsorption of CSF into the venous system. A decrease in the absorption of CSF through the arachnoid villi would lead to an accumulation of CSF in the ventricles, resulting in hydrocephalus. This is consistent with the patient's symptoms and physical examination findings.

**Why Each Wrong Option is Incorrect**
* **Option A:** The aqueduct of Sylvius is a narrow channel connecting the third and fourth ventricles. Obstruction of the aqueduct would lead to hydrocephalus, but it would be due to an obstruction of CSF flow, not a decrease in absorption.
* **Option B:** The foramen of Monro is a passageway connecting the lateral ventricles to the third ventricle. Obstruction of the foramen of Monro would lead to hydrocephalus, but it would be due to an obstruction of CSF flow, not a decrease in absorption.
* **Option D:** The cisterna magna is a subarachnoid space located between the cerebellum and the medulla oblongata. While obstruction of CSF flow through the cisterna magna could lead to hydrocephalus, it would not be due to a decrease in absorption.

**Clinical Pearl / High-Yield Fact**
The arachnoid villi are responsible for the reabsorption of approximately 500-600 mL of CSF per day, which is equivalent to about 20-25% of the total CSF production.

**Correct Answer:** C. Arachnoid villi.