What antidote should the nurse prepare to administer for a client undergoing a phenelzine-induced hypertensive crisis?

Phentolamine is the appropriate antidote to administer in a case of a phenelzine-induced hypertensive crisis. Phenelzine is a monoamine oxidase inhibitor (MAOI) that can lead to significant increases in blood pressure if the patient consumes tyramine-rich foods or certain medications, which are interactions that can cause a hypertensive crisis.

Phentolamine is a non-selective alpha-adrenergic antagonist that effectively lowers blood pressure by inhibiting the actions of catecholamines, leading to vasodilation and a decrease in blood vessel constriction. By administering phentolamine, the nurse can rapidly reverse the vasoconstrictor effects caused by excess norepinephrine, hence alleviating the hypertension associated with this kind of crisis.

In contrast, the other options are unsuitable for treating a hypertensive crisis induced by phenelzine. Atropine, an anticholinergic agent, is used to treat bradycardia and does not address hypertension. Naloxone is an opioid antagonist used for opioid overdose and is unrelated to the mechanisms involved in a hypertensive crisis from MAOIs. Flumazenil is a benzodiazepine antagonist, useful for reversing benzodiazepine sedation,