Which of the following drugs is NOT an MAOI?

Amitriptyline is the correct choice because it is a tricyclic antidepressant (TCA) rather than a monoamine oxidase inhibitor (MAOI). MAOIs function by inhibiting the enzyme monoamine oxidase, which is responsible for the breakdown of neurotransmitters such as serotonin, norepinephrine, and dopamine. This leads to an increase in the availability of these neurotransmitters, which can help improve mood and alleviate symptoms of depression.

In contrast, amitriptyline works primarily by blocking the reuptake of serotonin and norepinephrine at the synaptic level, thus increasing their levels in the synaptic cleft. While both MAOIs and amitriptyline are used to treat depression, their mechanisms of action are different, which places amitriptyline outside the category of MAOIs. The other drugs listed, selegiline, tranylcypromine, and phenelzine, are all classified as MAOIs due to their ability to inhibit the monoamine oxidase enzyme.