**Core Concept**
Selective serotonin reuptake inhibitors (SSRIs) are a class of medications primarily used to treat depression by increasing the levels of serotonin in the synaptic cleft. This is achieved through inhibition of the serotonin transporter, preventing the reuptake of serotonin by the presynaptic neuron.

**Why the Correct Answer is Right**
SSRIs are considered first-line treatment for depression due to their efficacy and relatively favorable side effect profile compared to other antidepressants. The mechanism of action involves the inhibition of the serotonin transporter (SERT), which increases the concentration of serotonin in the synaptic cleft and enhances neurotransmission. This leads to improved mood, reduced symptoms of depression, and enhanced overall quality of life.

**Why Each Wrong Option is Incorrect**
**Option A:** SSRIs are not the first-line treatment for anxiety disorders, although they can be used as adjunctive therapy. They are primarily used to treat depression.
**Option B:** Antipsychotics are not typically used as first-line treatment for depression, although they can be used in combination with SSRIs in treatment-resistant cases.
**Option C:** Tricyclic antidepressants (TCAs) are not considered first-line treatment for depression due to their side effect profile and potential toxicity.

**Clinical Pearl / High-Yield Fact**
SSRIs can be effective in treating depression with a relatively favorable side effect profile, but it's essential to monitor patients for potential interactions with other medications and side effects such as serotonin syndrome.

**Correct Answer:** D. Major Depressive Disorder.