Which of the following is an example of a histamine blocker?

Zyrtec is a well-known example of a histamine blocker, specifically classified as an antihistamine. Antihistamines work by inhibiting the action of histamine, a chemical released during allergic reactions that can lead to symptoms such as itching, sneezing, and nasal congestion. By blocking the H1 receptors, Zyrtec alleviates these allergic symptoms, making it effective for conditions like seasonal allergies and hay fever.

Other options provided represent different classes of medications that address various conditions but do not function as histamine blockers. For example, Singulair is utilized for asthma and allergic rhinitis but acts as a leukotriene receptor antagonist. Cromolyn is a mast cell stabilizer that prevents the release of histamine rather than blocking its action. Prednisone is a corticosteroid that reduces inflammation and suppresses the immune response, which is different from the mechanism of antihistamines. Understanding these distinct actions helps clarify why Zyrtec is considered the correct option for this question.