What is phlegm?

Phlegm refers specifically to the thick mucus produced in the respiratory passages, particularly when the body is experiencing an infection or irritation. This substance plays a crucial role in the body's defense mechanism, helping to trap and eliminate pathogens, dust, and other foreign particles from the airways.

When an individual has a respiratory condition such as a cold, bronchitis, or another illness, the body tends to produce more phlegm as a response to clear the infection. The viscosity of phlegm distinguishes it from regular mucus, which is generally thinner and found in the nasal passages.

Understanding the nature of phlegm is vital for healthcare providers as it can give clues about a patient's condition. For instance, a change in the color or consistency of phlegm may indicate a specific type of infection or other underlying health issues. Thus, B is the correct choice as it accurately describes the nature of phlegm, which directly correlates to its function in the respiratory system.