**Core Concept:** Pulsus Paradoxus is a physiologic finding in patients with pulmonary hypertension where a decrease in systolic blood pressure is seen during expiration due to increased left atrial pressure.

**Why the Correct Answer is Right:** In this scenario, the patient develops sudden hypotension, raised central venous pressure, and pulsus paradoxus at the 4th hour postoperatively following cardiac surgery. These signs indicate that the patient has developed pulmonary hypertension due to increased left atrial pressure.

**Why Each Wrong Option is Incorrect:**

A. Pulmonary Embolism (PE) typically presents with acute onset chest pain, tachypnea, and hypoxia, not hypotension, raised CVP, and pulsus paradoxus.
B. Pulmonary Embolism is a common cause of acute respiratory distress syndrome (ARDS), but it usually presents with acute onset symptoms, not the progressive symptoms observed in this case.
C. Cor pulmonale is a chronic condition caused by pulmonary hypertension, presenting with chronic exertional dyspnea and clubbing, not the sudden onset symptoms observed in this case.
D. Cardiac tamponade is characterized by hypotension and altered jugular venous pressure, not the progressive symptoms observed in this case.

**Clinical Pearl:** Pulsus Paradoxus is a valuable clinical sign in detecting pulmonary hypertension, particularly in the context of post-cardiac surgery patients. It alerts clinicians to the possibility of complications like pulmonary embolism, cor pulmonale, or cardiac tamponade.

**Correct Answer:** D. Cardiac tamponade is the correct answer as it presents with sudden onset symptoms, progressive symptoms, and pulsus paradoxus. Pulmonary embolism, cor pulmonale, and cardiac tamponade are not consistent with the presented clinical picture.