## Core Concept
Selective serotonin reuptake inhibitors (SSRIs) are a class of drugs primarily used as antidepressants in the treatment of major depressive disorder and anxiety disorders. They work by selectively inhibiting the reuptake of serotonin (5-HT) in the brain, which increases the level of serotonin available in the synaptic cleft. This mechanism is key to their therapeutic effects.

## Why the Correct Answer is Right
Mirtazapine is classified as a noradrenergic and specific serotonergic antidepressant (NaSSA). It works by antagonizing central presynaptic α2-adrenergic inhibitory autoreceptors and heteroreceptors, which increases the release of norepinephrine and serotonin. Unlike SSRIs, mirtazapine's mechanism does not primarily involve the selective inhibition of serotonin reuptake.

## Why Each Wrong Option is Incorrect
* **Option A:** Fluoxetine is a well-known SSRI used in the treatment of depression, anxiety disorders, and some other conditions. It selectively inhibits serotonin reuptake.
* **Option B:** Sertraline is another SSRI that functions by inhibiting the reuptake of serotonin, thereby increasing its availability in the synaptic cleft.
* **Option D:** Paroxetine is also an SSRI, used for treating depression and various anxiety disorders, and works through selective serotonin reuptake inhibition.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is to recognize that while SSRIs are commonly used for depression and anxiety, their side effect profiles and specific indications can vary. For example, paroxetine is often used for social anxiety disorder, and sertraline is used for post-traumatic stress disorder (PTSD). Mirtazapine, on the other hand, is noted for its efficacy in patients with major depressive disorder who have not responded to SSRIs and for its antiemetic properties which can help with nausea and weight gain.

## Correct Answer: . Mirtazapine