Which of the following is classified as a monoamine oxidase inhibitor (MAOI)?

Phenelzine is classified as a monoamine oxidase inhibitor (MAOI) because it works by inhibiting the enzyme monoamine oxidase, which is responsible for breaking down neurotransmitters such as serotonin, norepinephrine, and dopamine in the brain. By inhibiting this enzyme, phenelzine increases the levels of these neurotransmitters, which can help improve mood and alleviate symptoms of depression. This mechanism is distinct from other classes of antidepressants, such as selective serotonin reuptake inhibitors (SSRIs) or norepinephrine-dopamine reuptake inhibitors (NDRIs), which act on neurotransmitter reuptake rather than degradation. Therefore, phenelzine is specifically recognized for its role as an MAOI in the treatment of depressive disorders.