Which medical condition is characterized by the accumulation of fluid in the alveoli, leading to impaired gas exchange?

The condition characterized by the accumulation of fluid in the alveoli, which directly impairs gas exchange, is pulmonary edema. In pulmonary edema, excess fluid enters the alveolar spaces, preventing oxygen from effectively diffusing into the bloodstream and carbon dioxide from being expelled. This fluid accumulation can be caused by several factors, such as heart failure, where increased hydrostatic pressure causes fluid to leak from blood vessels into the lung interstitium and ultimately the alveoli, or by direct lung injuries.

While other conditions listed can affect lung function and gas exchange, they do not specifically involve the accumulation of fluid within alveoli as their primary pathological feature. For instance, pneumonia involves infection and inflammation in the lung tissue, leading to consolidation but not primarily driven by fluid accumulation. Chronic obstructive pulmonary disease (COPD) typically features airway obstruction and chronic inflammation, but fluid accumulation in the alveoli is not the defining characteristic. Atelectasis, which is the collapse of lung tissue leading to impaired gas exchange, is also different as it involves a loss of air rather than fluid accumulation.

Thus, pulmonary edema is the most accurate and specific answer regarding the condition defined by the accumulation of fluid in the alveoli.