Final answer:
A collapsed lung, or pneumothorax, can be caused by factors such as abnormal connective tissue, emphysema from smoking, and certain restrictive diseases that collapse the airways.
Step-by-step explanation:
A collapsed lung, also known as pneumothorax, can be caused by various factors. One common cause is spontaneous pneumothorax, which is believed to be related to subtle abnormalities in connective tissue. Another cause is emphysema, a lung disease usually caused by smoking, in which the walls of the alveoli break down, reducing gas exchange in the lungs. Additionally, certain restrictive diseases can cause the airways to collapse, trapping air in the lungs.