Increased airway resistance is/are caused by
**Core Concept**
The question is testing the understanding of factors influencing airway resistance, a critical concept in respiratory physiology. Airway resistance is the opposition to airflow in the airways, and it can be affected by various factors including anatomical, physiological, and pathological conditions.
**Why the Correct Answer is Right**
Airway resistance is increased by conditions that narrow the airway lumen, such as bronchoconstriction. This occurs when smooth muscle in the airway wall contracts, reducing the diameter of the airway. This is mediated by the release of bronchoconstrictor substances such as histamine, acetylcholine, and leukotrienes from various cells, including mast cells, eosinophils, and epithelial cells. These substances stimulate muscarinic receptors on the smooth muscle cells, leading to contraction and increased airway resistance.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because increased airway resistance is not typically caused by increased airway diameter. While conditions such as pulmonary edema can increase airway resistance, it is not due to an increase in airway diameter.
**Option B:** Incorrect because increased airway resistance is not typically caused by decreased airway pressure. While airway pressure can affect airflow, increased airway resistance is more directly related to anatomical and physiological factors.
**Option C:** Incorrect because increased airway resistance is not typically caused by increased lung volume. While conditions such as hyperinflation can affect lung mechanics, increased airway resistance is more directly related to airway constriction and narrowing.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that bronchoconstriction is a critical factor in increased airway resistance, and it can be mediated by various substances released from different cells. This is a common mechanism underlying conditions such as asthma, chronic obstructive pulmonary disease (COPD), and bronchitis.
**Correct Answer: D. Bronchoconstriction**