**Core Concept**
Unipolar depression is a type of major depressive disorder characterized by persistent and intense feelings of sadness, hopelessness, and loss of interest in activities. The treatment for unipolar depression typically involves a combination of pharmacological and non-pharmacological interventions.

**Why the Correct Answer is Right**
The first-line treatment for unipolar depression often includes selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, sertraline, or paroxetine. These medications work by inhibiting the reuptake of serotonin, increasing its availability in the synaptic cleft, and thereby enhancing serotonergic neurotransmission. This mechanism of action is thought to be responsible for the antidepressant effects of SSRIs.

**Why Each Wrong Option is Incorrect**
**Option A:** Tricyclic antidepressants (TCAs) are another class of medications used to treat depression, but they are generally considered to be less effective and more side-effect prone than SSRIs.
**Option B:** Monoamine oxidase inhibitors (MAOIs) are a third class of antidepressants, but they are typically reserved for use in patients who have not responded to other treatments due to their potential for interactions with other medications and foods.
**Option C:** Lithium is a mood stabilizer primarily used to treat bipolar disorder, although it may also be used to treat unipolar depression in some cases. However, it is not typically considered a first-line treatment for unipolar depression.

**Clinical Pearl / High-Yield Fact**
SSRIs are generally considered to be safer and more effective than other classes of antidepressants, but they may take several weeks to produce noticeable effects. It is essential to monitor patients for signs of suicidal ideation, particularly during the initial stages of treatment.

**Correct Answer: D. SSRIs**