Which of the following is NOT a common Selective Serotonin Reuptake Inhibitor (SSRI)?

Amitriptyline is not classified as a Selective Serotonin Reuptake Inhibitor (SSRI); rather, it is a tricyclic antidepressant (TCA). SSRIs, including citalopram, fluoxetine, and sertraline, function primarily by blocking the reuptake of serotonin in the brain, which increases its availability and leads to improvements in mood and anxiety symptoms.

Citalopram, fluoxetine, and sertraline are all well-known SSRIs used widely in clinical practice for the treatment of depression and anxiety disorders. Each of these medications can also have varying effects on other neurotransmitters, but their primary mechanism of action is serotonin reuptake inhibition. In contrast, amitriptyline works on multiple neurotransmitter systems, including serotonin and norepinephrine, but it does so in a fundamentally different manner than SSRIs, which is why it doesn't belong to this specific class of medications.