Which vasoconstrictor is contraindicated with concomitant MAO inhibitor therapy?

The vasoconstrictor that is contraindicated with concomitant MAO inhibitor therapy is phenylephrine. This is primarily due to the fact that monoamine oxidase (MAO) inhibitors can significantly enhance the effects of sympathomimetic amines, like phenylephrine. MAO is an enzyme responsible for the breakdown of various neurotransmitters, including catecholamines. When a patient is taking MAO inhibitors, there is a decrease in the metabolism of these neurotransmitters, leading to an increased risk of hypertensive crisis when a vasoconstrictor that relies on the sympathetic nervous system's functioning, such as phenylephrine, is used.

In contrast, norepinephrine and epinephrine also exert vasoconstrictor effects but are less likely to cause significant interactions with MAO inhibitors in the same way as phenylephrine. Levonordefrin, while also a vasoconstrictor, has a different pharmacological profile and is not commonly associated with this specific contraindication when used in patients on MAO therapy. This further supports the conclusion that phenylephrine poses a distinct risk in the context of MAO inhibitors.