Which condition is characterized by a mass adjacent to the right atrium that does not communicate with the pericardium?

The correct answer is pericardial cyst, which is a benign, fluid-filled sac that can occur adjacent to the right atrium. Pericardial cysts typically arise from a developmental abnormality of the pericardium and do not communicate with the pericardial space, which sets them apart from other conditions that might involve fluid around the heart.

Pericardial effusions, in contrast, involve the accumulation of fluid in the pericardial space and do communicate with the pericardium, making them unsuitable for this description. Cardiac tamponade is a clinical syndrome resulting from increased pressure in the pericardial space, often due to effusion, and likewise does not pertain to a mass adjacent to the atrium. Constrictive pericarditis is characterized by thickening and fibrotic changes of the pericardium that restrict diastolic filling but does not define a mass adjacent to the right atrium. Thus, the nature of a pericardial cyst aligns perfectly with the criteria outlined in the question.