Step 1: Know the classes of antidepressants.
Antidepressants are grouped by mechanism: SSRIs (selective serotonin reuptake inhibitors), TCAs (tricyclic antidepressants), MAOIs (monoamine oxidase inhibitors), and SNRIs. Each class has distinct chemistry and pharmacology.
Step 2: What is phenelzine?
Phenelzine is a hydrazine derivative. It irreversibly inhibits the enzyme monoamine oxidase (MAO), which normally breaks down neurotransmitters like serotonin, norepinephrine, and dopamine. By blocking MAO, phenelzine raises the levels of these monoamines in the brain.
Step 3: Classify correctly.
Since phenelzine works by inhibiting MAO, it is a Monoamine Oxidase Inhibitor (MAOI). It is one of the older antidepressants used when other drugs fail.
Step 4: Eliminate distractors.
SSRIs (like fluoxetine) block serotonin reuptake transporters. Benzodiazepines are not antidepressants; they are anxiolytics. TCAs (like amitriptyline) block amine reuptake via a tricyclic structure. Phenelzine fits none of these.
Step 5: Confirm.
Phenelzine (Nardil) is the classic MAOI antidepressant alongside tranylcypromine and isocarboxazid.
Answer: Option (4) — Monoamine Oxidase Inhibitors