**Core Concept**
Fetal ventriculomegaly refers to an abnormal enlargement of the cerebral ventricles in the developing fetus, which can be associated with various underlying causes. This condition is often identified through prenatal ultrasound and is a significant concern due to its potential impact on fetal development and long-term neurological outcomes.

**Why the Correct Answer is Right**
The most common cause of fetal ventriculomegaly is obstructive hydrocephalus, often resulting from aqueductal stenosis. Aqueductal stenosis is a congenital narrowing of the cerebral aqueduct, which is a narrow channel connecting the third and fourth ventricles. This narrowing leads to an obstruction of cerebrospinal fluid (CSF) flow, causing an accumulation of CSF in the ventricles and subsequent ventriculomegaly. The obstruction can also lead to increased intracranial pressure, which may further exacerbate ventriculomegaly.

**Why Each Wrong Option is Incorrect**
* **Option A:** Dandy-Walker malformation is another cause of fetal ventriculomegaly, but it is less common than aqueductal stenosis. Dandy-Walker malformation involves a congenital cystic dilation of the fourth ventricle and an abnormal development of the cerebellum.
* **Option B:** Infection, such as congenital toxoplasmosis or cytomegalovirus, can also cause fetal ventriculomegaly, but it is not the most common cause.
* **Option D:** Other causes of fetal ventriculomegaly, such as posterior fossa tumors or Chiari II malformation, are rare and not as common as aqueductal stenosis.

**Clinical Pearl / High-Yield Fact**
Fetal ventriculomegaly is a significant concern due to its potential association with long-term neurological outcomes, including developmental delay, seizures, and intellectual disability. Early identification and management are crucial to optimize fetal outcomes.

**Correct Answer: C. Aqueductal stenosis.