An elderly man has ingested a large quantity of prazosin tablets, a drug that blocks α1 adrenergic receptors. Which symptom is most likely to be seen in this patient?

Ingestion of a large quantity of prazosin, an alpha-1 adrenergic receptor blocker, is likely to result in significant vasodilation. Prazosin primarily acts on these receptors in vascular smooth muscle, leading to relaxation of blood vessels. This effect reduces systemic vascular resistance and can result in decreased blood pressure. Therefore, the dilation of blood vessels (vasorelaxation) is a prominent consequence of prazosin overdose, making it the most likely symptom to be observed in this patient.

While reduced heart rate could occur in the context of baroreceptor reflex mechanisms when blood pressure drops, it is less likely to be a direct symptom of prazosin's action. Increased blood pressure contradicts the pharmacological effect of prazosin, which is intended to lower blood pressure through vasodilation. Reduction in urinary frequency would also not align with the effects of prazosin, as it is commonly used to treat urinary retention symptoms associated with benign prostatic hyperplasia by relaxing smooth muscle in the bladder neck as well. Hence, the most characteristic response observed in this case is vasorelaxation due to prazosin's mechanism of action.