Which of the following agents is most likely to cause sedating side effects?

A. Fexofenadine: Fexofenadine is a second-generation antihistamine that selectively blocks peripheral H1 receptors without significant penetration of the blood-brain barrier. As a result, it has minimal central nervous system effects and rarely causes sedation, making it safe for daytime use. B. Pseudoephedrine: Pseudoephedrine is a systemic sympathomimetic decongestant that stimulates alpha-adrenergic receptors, causing vasoconstriction of nasal blood vessels. Its primary side effects include insomnia, jitteriness, and increased heart rate, but it is not sedating. C. Diphenhydramine: Diphenhydramine is a first-generation antihistamine that crosses the blood-brain barrier and blocks central H1 receptors. This action leads to significant sedation, drowsiness, and impaired cognitive function, making it the most sedating option among the choices and often used as a sleep aid. D. Loratadine: Loratadine is another second-generation antihistamine with minimal central nervous system penetration. It provides relief from allergic symptoms without causing significant sedation, making it preferable for daytime allergy management.