**Core Concept**
The aqueduct of Sylvius is a narrow passage connecting the third and fourth ventricles in the brain, allowing cerebrospinal fluid (CSF) to flow from the third to the fourth ventricle. Obstruction at this site impairs the normal circulation of CSF, leading to changes in ventricular size.

**Why the Correct Answer is Right**
When the aqueduct of Sylvius is obstructed, CSF accumulates in the third ventricle due to impaired flow. As a result, the third ventricle expands. Additionally, the increased pressure in the third ventricle causes the lateral ventricles to dilate, as they are connected to the third ventricle via the foramina of Monro. This leads to enlargement of both the lateral and third ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because obstruction at the aqueduct of Sylvius does not affect the fourth ventricle directly. The fourth ventricle is connected to the subarachnoid space and does not have a direct communication with the third ventricle.
**Option B:** This option is incorrect because, although the lateral ventricles are affected, they are not the only ventricles to be enlarged. The third ventricle is also significantly impacted by the obstruction.
**Option C:** This choice is incorrect because the fourth ventricle is not directly affected by the obstruction at the aqueduct of Sylvius. The CSF flow into the fourth ventricle remains intact.

**Clinical Pearl / High-Yield Fact**
In cases of aqueductal stenosis or obstruction, the third and lateral ventricles are disproportionately enlarged, often leading to increased intracranial pressure and potentially life-threatening complications. This condition is commonly associated with hydrocephalus, a condition characterized by the accumulation of CSF within the brain's ventricles.

**✓ Correct Answer: D. Both lateral and third ventricles**