What condition is Diphenhydramine primarily used to relieve?

Diphenhydramine is primarily used to relieve allergy symptoms due to its classification as an antihistamine. It works by blocking the action of histamine, a substance in the body that can cause allergic symptoms such as sneezing, itching, runny nose, and hives. This makes it particularly effective for treating various allergic reactions, including hay fever and other types of allergic rhinitis.

In the context of the options provided, the other conditions listed are not treated effectively with Diphenhydramine. For instance, hypertension is managed with medications that specifically lower blood pressure, while shock requires immediate interventions that can include fluids, medications, and other critical care measures. Heart failure involves a complex management strategy involving diuretics, ACE inhibitors, and other drugs tailored specifically to support heart function. Therefore, the use of Diphenhydramine as an antihistamine makes it the appropriate choice when addressing allergy symptoms.