**Core Concept**
Asthma is a chronic inflammatory disease characterized by airway hyperresponsiveness, reversible airflow obstruction, and a tendency to bronchospasm. The universal finding in asthma is related to the pathophysiological changes that occur in the airways.

**Why the Correct Answer is Right**
The correct answer is due to the presence of airway inflammation, characterized by the infiltration of eosinophils, neutrophils, and lymphocytes into the airway wall. This leads to the production of various inflammatory mediators, such as histamine, leukotrienes, and cytokines, which contribute to the development of airway hyperresponsiveness. The airway smooth muscle thickening and mucus gland hyperplasia are also key features of asthma.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because asthma is not primarily characterized by a decrease in airway diameter. While bronchospasm can occur in asthma, it is not the universal finding.
* **Option B:** This option is incorrect because airway resistance is increased in asthma, not decreased. The airway narrowing and airway smooth muscle thickening contribute to the increased resistance.
* **Option C:** This option is incorrect because airway edema is a feature of asthma, but it is not the universal finding. The airway inflammation and airway smooth muscle thickening are more characteristic of asthma.

**Clinical Pearl / High-Yield Fact**
A key feature of asthma is airway hyperresponsiveness, which can be triggered by a variety of stimuli, including allergens, irritants, and respiratory infections. Understanding the pathophysiological changes in the airways is essential for the management of asthma.

**Correct Answer:** D.