**Core Concept**
The inspiratory and expiratory loops in the pressure-volume curve of the lungs are shaped by the elastic and resistive properties of the respiratory system. The main difference in trajectory between the two loops is due to the hysteresis phenomenon, which arises from the nonlinear and time-dependent behavior of lung tissues and airways.

**Why the Correct Answer is Right**
The inspiratory loop represents the loading of the lung with air, whereas the expiratory loop represents the unloading of the lung. The hysteresis between the two loops is caused by the viscoelastic properties of lung tissue, which is more compliant during inflation (inspiration) than during deflation (expiration). This results in a greater pressure required to deflate the lung compared to inflating it, leading to the characteristic shape of the pressure-volume curve.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the fundamental reason for the difference in trajectory between the inspiratory and expiratory loops.

**Option B:** This option is incorrect because it focuses on the elastic properties of the lung, which only partially explain the hysteresis phenomenon.

**Option C:** This option is incorrect because it mentions resistance, but resistance alone does not account for the nonlinear behavior of the lung during inflation and deflation.

**Clinical Pearl / High-Yield Fact**
The hysteresis phenomenon in the pressure-volume curve of the lung is a critical factor in understanding respiratory mechanics and the pathophysiology of lung diseases. Recognizing the difference in trajectory between the inspiratory and expiratory loops is essential for diagnosing conditions such as pulmonary fibrosis, where the lung becomes stiffer and more resistant to deflation.

**Correct Answer: D. Hysteresis due to viscoelastic properties of lung tissue.**