**Core Concept**
Pulsus paradoxus is a clinical phenomenon characterized by a greater than 10 mmHg decrease in systolic blood pressure during inspiration. This occurs due to an exaggerated decrease in intrathoracic pressure during inspiration, leading to a relative decrease in venous return to the heart.

**Why the Correct Answer is Right**
In patients with emphysema or asthma, the lungs overinflate and become less compliant, leading to an increase in intra-thoracic pressure. During inspiration, the increased negative pressure within the thoracic cavity causes a greater decrease in intrathoracic pressure, which in turn results in a more pronounced decrease in venous return to the heart. This decrease in venous return leads to a relative decrease in cardiac output, and subsequently, a greater than 10 mmHg decrease in systolic blood pressure during inspiration, resulting in pulsus paradoxus.

**Why Each Wrong Option is Incorrect**
**Option A:** Shock can indeed cause pulsus paradoxus, but it is not the most specific or characteristic condition associated with this phenomenon. Other factors such as cardiac tamponade, constrictive pericarditis, and severe asthma or COPD are more directly linked to pulsus paradoxus.
**Option B:** Age is not a direct factor in the development of pulsus paradoxus. While elderly patients may have underlying conditions that increase the likelihood of pulsus paradoxus, age alone is not a determining factor.
**Option D:** High output states, such as sepsis or anemia, typically do not cause pulsus paradoxus. In fact, these conditions often result in a compensatory increase in cardiac output, which would counteract the development of pulsus paradoxus.

**Clinical Pearl / High-Yield Fact**
Pulsus paradoxus can be remembered using the mnemonic "P-A-S-S": P - Prolonged expiration, A - Asthma, S - Severe COPD, S - Severe cardiac conditions.

**✓ Correct Answer: C. Emphysema/Asthma**