A 63-year-old man presents to the Emergency Department with progressive breathlessness and chest discomfort. He is tachycardic and uncomfortable but remains normotensive. Bedside echocardiography shows a moderate circumferential pericardial effusion. The left-sided chambers appear preserved, and the heart is not freely oscillating within the effusion. You are looking for early two-dimensional echocardiographic evidence of evolving tamponade physiology.

Which echocardiographic finding is most consistent with early cardiac tamponade? Select only one best single option.