Which condition is characterized by stiff lungs?

Acute respiratory distress syndrome (ARDS) is characterized by stiff lungs due to an inflammatory process that leads to increased lung permeability, resulting in pulmonary edema and impaired gas exchange. In ARDS, the alveoli (air sacs in the lungs) become filled with fluid, and the normal compliance of the lung is reduced, making it harder for the patient to inhale and exhale effectively. This stiffness hampers ventilation and can lead to severe respiratory failure.

In contrast, conditions like pneumothorax involve the pleural space and result in collapsed lung tissue, which does not create stiffness in the same way as ARDS. Asthma is primarily characterized by airway obstruction and bronchoconstriction, leading to wheezing and difficulty in breathing, but does not inherently cause the stiffness associated with lung compliance. Chronic bronchitis involves mucus production and airway inflammation but primarily affects airflow rather than the lung's structural compliance. Thus, ARDS is distinct in its mechanism of causing lung stiffness, making it the correct answer.