**Core Concept**
Aqueductal stenosis and Dandy-Walker malformation are two distinct causes of hydrocephalus, a condition characterized by an accumulation of cerebrospinal fluid (CSF) in the brain, leading to increased intracranial pressure. Understanding the differences between these two conditions is crucial for accurate diagnosis and treatment.

**Why the Correct Answer is Right**
Aqueductal stenosis involves the narrowing of the aqueduct of Sylvius, a small passageway in the midbrain that allows CSF to flow from the third ventricle to the fourth ventricle. This stenosis leads to an accumulation of CSF in the lateral ventricles, causing hydrocephalus. In contrast, Dandy-Walker malformation is a congenital disorder characterized by the hypoplasia (underdevelopment) of the cerebellum, with or without cystic dilatation of the fourth ventricle. The key distinguishing feature between the two conditions is the presence of a cystic dilatation of the fourth ventricle in Dandy-Walker malformation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it does not specifically distinguish between aqueductal stenosis and Dandy-Walker malformation. Both conditions can present with hydrocephalus, so this option is not specific enough.

**Option B:** While this option might seem plausible, it is not a distinguishing feature between the two conditions. Both aqueductal stenosis and Dandy-Walker malformation can present with increased intracranial pressure and hydrocephalus, so this option is not specific enough.

**Option C:** This option is incorrect because both conditions can present with enlarged ventricles, so this is not a distinguishing feature between the two.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Dandy-Walker malformation is often associated with other congenital anomalies, such as Chiari II malformation and syringomyelia. In contrast, aqueductal stenosis is typically an isolated condition.

**Correct Answer:** D. The presence of a cystic dilatation of the fourth ventricle is a key distinguishing feature between aqueductal stenosis and Dandy-Walker malformation.