A 32-Year-Old Woman Comes to the Emergency Department Due to Severe

A 32-year-old woman comes to the emergency department due to severe headache, nausea, and vomiting for the past several hours.  The headache is pounding and localized to the left side.  She also describes bright spots in her vision prior to headache onset.  The patient has had similar episodes occasionally since adolescence.  Physical examination shows normal pupils and fundi, but the patient has discomfort to bright light.  There is no facial droop, extremity weakness, or sensory loss.  The patient is treated with metoclopramide and diphenhydramine.  Which of the following is the most likely reason diphenhydramine was used in this patient?

A) To decrease the risk of arrhythmia
B) To increase the prokinetic effect
C) To prevent abnormal motor reaction
D) To prevent allergic reaction
E) To prevent rebound headache