**Core Concept**
Hydrocephalus ex vacuo refers to the enlargement of ventricles and an increase in cerebrospinal fluid (CSF) volume that occurs in the setting of brain atrophy, often due to aging or neurodegenerative diseases. This phenomenon is characterized by the compensatory expansion of ventricular spaces in response to the loss of brain tissue.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of brain atrophy and its effects on ventricular size and CSF volume. As brain tissue degenerates, the ventricles expand to accommodate the increased volume of CSF. This compensatory mechanism helps to maintain normal intracranial pressure by allowing the ventricles to increase in size and accommodate the lost brain tissue. The key anatomical structure involved in this process is the ventricular system, which includes the lateral ventricles, third ventricle, and fourth ventricle.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as this is the correct answer.
* **Option B:** This option is incorrect because it does not accurately describe the compensatory mechanism occurring in hydrocephalus ex vacuo. While brain tumors can cause hydrocephalus, the underlying mechanism is different from the compensatory enlargement seen in brain atrophy.
* **Option C:** This option is incorrect because it does not accurately describe the relationship between brain atrophy and ventricular size. While brain injury can cause hydrocephalus, the underlying mechanism is different from the compensatory enlargement seen in brain atrophy.
* **Option D:** This option is incorrect because it does not accurately describe the compensatory mechanism occurring in hydrocephalus ex vacuo. While hydrocephalus can be caused by various factors, the correct answer is specific to the compensatory enlargement of ventricles due to brain atrophy.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that hydrocephalus ex vacuo is a common finding in elderly patients with brain atrophy, and it can mimic the symptoms of other neurological conditions. Therefore, it is essential to consider this diagnosis in patients with cognitive decline, gait disturbance, or other neurological symptoms, especially in the absence of other obvious causes of hydrocephalus.

**Correct Answer: C. Normal pressure hydrocephalus.**