**Core Concept**
Pericardial effusion and cardiac dilatation are two distinct conditions that can lead to similar clinical presentations, making it essential to differentiate between them. The investigation of choice should provide accurate measurements of the pericardial space and cardiac chamber sizes.

**Why the Correct Answer is Right**
The investigation of choice for differentiating pericardial effusion and cardiac dilatation is Echocardiography. Echocardiography uses ultrasound waves to produce images of the heart and pericardium, allowing for the measurement of the pericardial space and cardiac chamber sizes. It can accurately diagnose pericardial effusion by demonstrating an increased pericardial space, and differentiate it from cardiac dilatation by showing an enlarged cardiac chamber size without an increased pericardial space. The Doppler mode can also be used to assess the flow of blood through the heart chambers and pericardium, further aiding in the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not the investigation of choice for differentiating pericardial effusion and cardiac dilatation. While a chest X-ray can provide some information about the cardiac silhouette, it is not sensitive or specific enough to accurately diagnose pericardial effusion or cardiac dilatation.

**Option B:** Computed Tomography (CT) scan is not the investigation of choice for differentiating pericardial effusion and cardiac dilatation. While a CT scan can provide detailed images of the heart and pericardium, it is not as readily available or as cost-effective as echocardiography, and may expose patients to radiation.

**Option C:** Magnetic Resonance Imaging (MRI) is not the investigation of choice for differentiating pericardial effusion and cardiac dilatation. While MRI can provide detailed images of the heart and pericardium, it is not as readily available or as cost-effective as echocardiography, and may not be as sensitive for detecting pericardial effusion.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that echocardiography is a non-invasive, readily available, and cost-effective investigation that can accurately diagnose pericardial effusion and differentiate it from cardiac dilatation. A pericardial effusion is typically defined as an accumulation of more than 20 mL of fluid in the pericardial space.

**Correct Answer: C. Echocardiography. Echocardiography is the investigation of choice for differentiating pericardial effusion and cardiac dilatation.**