**Core Concept**
Leukotriene (LT) antagonists are a class of medications used in the management of asthma by modulating the leukotriene pathway, which plays a crucial role in the inflammatory process of asthma. These medications target the cysteinyl leukotriene receptors, thereby inhibiting the bronchoconstrictive, inflammatory, and chemotactic effects of leukotrienes.

**Why the Correct Answer is Right**
LT antagonists, such as montelukast, work by selectively blocking the cysteinyl leukotriene receptor 1 (CysLT1R) on airway smooth muscle cells, epithelial cells, and inflammatory cells. This blockade prevents the binding of leukotrienes to their receptors, leading to a reduction in bronchoconstriction, mucus production, and inflammation in the airways. The blockade also inhibits the recruitment of inflammatory cells, such as eosinophils, to the airways, thereby reducing airway inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because LT antagonists do not directly relax airway smooth muscle. While they may indirectly contribute to bronchodilation by reducing airway inflammation, their primary mechanism of action is not through direct smooth muscle relaxation.
**Option B:** This option is incorrect because LT antagonists do not increase the levels of surfactant in the lungs. Surfactant is a class of lipoproteins that reduce surface tension in the alveoli, but LT antagonists do not have a direct effect on surfactant production.
**Option C:** This option is incorrect because LT antagonists do not directly stimulate the release of histamine from mast cells. Histamine is a key mediator of allergic inflammation, but LT antagonists work by blocking the action of leukotrienes, not by stimulating the release of histamine.

**Clinical Pearl / High-Yield Fact**
LT antagonists are often used as add-on therapy in patients with persistent asthma who remain symptomatic despite treatment with inhaled corticosteroids. They can be particularly useful in patients with exercise-induced bronchospasm or those who experience nocturnal asthma symptoms.

**Correct Answer:** D.