Answer: Pneumonia.
Rationale: Atelectasis is "complete lung expansion or collapse of alveoli", preventing pressure changes and gas exchange by diffusion in the lungs. Conditions that predispose patients to atelectasis are obstructions of the airway by disease or condition that results in thickening of alveolar-capillary membranes, like pulmonary edema, which makes diffusion difficult. Stiffer lungs also tend to collapse and their alveoli also collapse.