Which of the following is NOT a cause of rapid-onset dyspnea?

Rapid-onset dyspnea typically refers to sudden shortness of breath that can arise from various acute medical conditions. Among the options provided, chronic asthma is characterized by a long-term inflammatory condition affecting the airways, which can lead to symptoms over time, particularly in response to triggers. While individuals with chronic asthma can experience acute exacerbations, the overall nature of asthma as a chronic disease means that it does not inherently lead to rapid onset symptoms without a specific acute trigger being involved.

In contrast, acute bronchospasm, anaphylaxis, and pneumothorax are all conditions that can result in a sudden and severe breathing difficulty. Acute bronchospasm typically occurs due to an immediate response to allergens or irritants, causing the airways to constrict rapidly. Anaphylaxis, a severe allergic reaction, can also lead to swift respiratory distress among other systemic symptoms. Pneumothorax is the presence of air in the pleural space that can occur suddenly and lead to immediate respiratory distress.

Therefore, chronic asthma is the option that lacks the characteristic of rapid onset, distinguishing it from the others that are directly associated with sudden dyspnea.