Final answer:
The echo-free space in a pericardial effusion is seen between the epicardium and the parietal serous pericardium, as this is where the pericardial cavity with the serous fluid is located.
Step-by-step explanation:
In patients with pericardial effusion, an echo-free space will be seen between the epicardium and the parietal serous pericardium. The pericardial cavity, which is filled with lubricating serous fluid, lies between these two structures. This cavity allows the heart to contract without friction and is bounded externally by the tough fibrous pericardium and internally by the serous pericardium which in turn has two layers: the parietal pericardium which adheres to the fibrous pericardium and the visceral pericardium, or epicardium, which rests on the myocardium, the heart's muscular layer. The endocardium is the innermost layer of the heart, lining the heart chambers, and thus would not be involved in a pericardial effusion. Therefore, the correct answer to the question is D. Parietal Serous Pericardium.