Final answer:
The condition with flattened hemidiaphragms and widened intercostal spaces is emphysema, caused by smoking and leading to irreversible alveolar damage.
Step-by-step explanation:
The condition characterized by "flattening" of the hemidiaphragms and widening of the intercostal space is emphysema. This chronic lung disease is primarily caused by smoking, which leads to damage of the alveolar walls and reduction in elastic recoil of the lungs. This results in overinflated alveoli and air trapping, causing a barrel-shaped chest and the characteristic X-ray findings of flattened diaphragms and increased intercostal space.
As for the other options, empyema is a collection of pus in the pleural cavity, atelectasis refers to collapsed lung tissue, and pneumonia involves the alveoli becoming filled with fluid. Therefore, these conditions do not present with flattening of the hemidiaphragms and intercostal space widening as described in emphysema.