**Core Concept**
The aqueduct of Sylvius is a narrow passageway in the brain that connects the third and fourth ventricles, allowing cerebrospinal fluid (CSF) to flow between them. Obstruction to this flow can lead to an accumulation of CSF in the third ventricle.

**Why the Correct Answer is Right**
The obstruction at the aqueduct of Sylvius will result in an increase in intraventricular pressure, causing the third ventricle to expand. This is because the CSF cannot flow freely from the third to the fourth ventricle, leading to a build-up of pressure and subsequent enlargement of the third ventricle. The hydrodynamic forces will cause the lateral and third ventricles to expand, but the fourth ventricle may become smaller due to the reduced flow of CSF.

**Why Each Wrong Option is Incorrect**
**Option A:** The fourth ventricle is less likely to be enlarged due to the obstruction at the aqueduct of Sylvius, as the CSF flow into the fourth ventricle is reduced.
**Option B:** The lateral ventricles may become slightly enlarged due to the increased pressure, but they are not the primary site of enlargement in this scenario.
**Option C:** The obstruction at the aqueduct of Sylvius does not directly affect the temporal horns of the lateral ventricles.

**Clinical Pearl / High-Yield Fact**
In cases of aqueductal stenosis, the patient may present with symptoms of hydrocephalus, including headache, nausea, vomiting, and gait disturbances. The diagnosis can be confirmed with imaging studies such as MRI or CT scans.

**Correct Answer:** C. Lateral ventricles