**Core Concept**
Mast cells play a crucial role in allergic reactions and inflammation by releasing histamine and leukotrienes. These mediators cause blood vessels to dilate, smooth muscle to contract, and mucous glands to secrete more mucus, leading to symptoms such as itching, swelling, and bronchospasm. To prevent these reactions, certain medications target mast cell function.

**Why the Correct Answer is Right**
Nedocromil sodium is a mast cell stabilizer that prevents the release of histamine and leukotrienes from mast cells. By inhibiting the degranulation of mast cells, nedocromil sodium effectively reduces the symptoms associated with allergic reactions. This mechanism of action is distinct from antihistamines, which only block the action of histamine after it has been released.

**Why Each Wrong Option is Incorrect**
**Option A:** Zileuton is a leukotriene synthesis inhibitor that prevents the formation of leukotrienes but does not prevent the release of histamine from mast cells.
**Option C:** Zafirlukast is a leukotriene receptor antagonist that blocks the action of leukotrienes on their receptors but does not prevent their release from mast cells.
**Option D:** Fexofenadine is a non-sedating antihistamine that blocks the action of histamine on its receptors but does not prevent its release from mast cells.

**Clinical Pearl / High-Yield Fact**
Mast cell stabilizers like nedocromil sodium are often used in the treatment of allergic conjunctivitis and asthma, as they can prevent the symptoms associated with allergic reactions. However, they do not provide immediate relief and are typically used as a preventative measure.

**✓ Correct Answer:** B. Nedocromil sodium