Which of the following is a common sign of asthma?

Cyanosis is a common sign of asthma. In asthma, the airways are constricted and inflamed, making it difficult for air to flow in and out effectively. This can lead to decreased oxygen levels in the blood, resulting in cyanosis, which is a bluish discoloration of the skin, particularly noticeable in the lips and fingertips. This occurs when the body tissues do not receive enough oxygen, which can happen during an asthma attack when breathing becomes labored.

In contrast, conditions like flail chest, pneumothorax, and hemothorax are associated with different types of trauma or lung pathology. Flail chest involves multiple rib fractures causing instability in the chest wall, pneumothorax refers to air in the pleural space leading to lung collapse, and hemothorax denotes blood accumulation in the pleura, all of which are not typical manifestations of asthma.