**Core Concept**
Pulsus paradoxus is a clinical phenomenon characterized by an exaggerated decrease in systolic blood pressure (≥ 10 mmHg) during inspiration, often associated with conditions that affect the respiratory system or cardiovascular system. The underlying mechanism involves altered intrathoracic pressure changes during breathing, which affect cardiac output and venous return.

**Why the Correct Answer is Right**
Pulsus paradoxus is typically observed in conditions that increase the ratio of intrapleural pressure to intrathoracic pressure, such as severe asthma, chronic obstructive pulmonary disease (COPD), cardiac tamponade, and constrictive pericarditis. These conditions lead to increased intrathoracic pressure during inspiration, reducing venous return and subsequently cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Constrictive pericarditis is a known cause of pulsus paradoxus due to the rigid pericardium restricting cardiac filling during inspiration.

**Option B:** Cardiac tamponade is a classic cause of pulsus paradoxus, as the accumulation of fluid in the pericardial sac increases intrapericardial pressure, impeding cardiac filling during inspiration.

**Option C:** Severe asthma is a well-documented cause of pulsus paradoxus, as the airway obstruction increases intrathoracic pressure and reduces venous return during inspiration.

**Clinical Pearl / High-Yield Fact**
Pulsus paradoxus can be a useful clinical sign in diagnosing conditions that affect the respiratory or cardiovascular systems. However, it is essential to consider other clinical findings, such as the presence of wheezing or a pericardial rub, to narrow down the differential diagnosis.

**Correct Answer:** D.