A 41-year-old woman presents to the emergency depament with chest pain, shoness of breath, and worsening fatigue for the past day. The chest pain initially worsened with lying down and improved with leaning forward, but now it seems equal in intensity over all positions. On examination she has labored, fast breathing and appears to be in pain. She has jugular venous distention. She is tachycardic, and has distant hea sounds with a friction rub. Her lungs are clear to auscultation bilaterally, and she has no limb edema. Her pulse is 126/min, BP is 89/66 mm Hg, respiratory rate is 32/min, and oxygen saturation is 98% on room air. X-ray of the chest is shown below. Which of the following is the most likely diagnosis?

CARDIAC TAMPONADE: Evidence of cardiomegaly on CXR is due to a large pericardial effusion, and an echocardiogram would show a large pleural effusion with chamber collapse, a characteristic echocardiographic sign of tamponade. Moreover, this patient has the characteristic symptoms and signs of cardiac tamponade. She complains of chest pain, fatigue, and dyspnea, all characteristic of tamponade. On physical examination, she has Beck's triad, a group of signs characterized by hypotension, distant hea sounds, and dis- tended neck veins. The fluid accumulation around the hea decreases the ventricular filling pressure, which decreases cardiac output. She also has tachycardia and tachypnea, both found in patients with cardiac tamponade.