Final answer:
The patient's syncope is likely caused by the side effects of Valium and antihistamine use, which can lead to orthostatic hypotension, a condition where blood pressure drops upon standing, resulting in fainting.
Step-by-step explanation:
The possible cause of the patient's syncope is most likely related to a side effect of the medications they are taking. While bradycardia has been ruled out, both Valium (a benzodiazepine) and antihistamines have the potential to cause drowsiness, dizziness, and may contribute to orthostatic hypotension, which can result in syncope. Valium acts as a central nervous system depressant and can reduce blood pressure, while antihistamines can also cause sedation and may interfere with the autonomic regulation of blood pressure. When these medications are combined, the risk for orthostatic hypotension and consequently syncope may be increased. Consulting a healthcare provider prior to combining medications could help to avoid such adverse effects.