**Core Concept**
Lithium is a mood stabilizer used in the treatment of bipolar disorder, primarily due to its ability to modulate the activity of various neurotransmitters and its effects on the central nervous system.

**Why the Correct Answer is Right**
Lithium exerts its therapeutic effects by inhibiting the reuptake of serotonin and norepinephrine, thereby increasing their concentrations in the synaptic cleft. It also has a neuroprotective effect, possibly by reducing the excitotoxicity of glutamate and modulating the activity of GSK-3β, a key enzyme involved in the signaling pathways of various neurotransmitters. Additionally, lithium has a mood-stabilizing effect by reducing the amplitude of the manic episodes in bipolar disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** Lithium is not primarily used in the treatment of schizophrenia. While it may be used as an adjunct therapy in some cases, its primary indication is for the treatment of bipolar disorder.
**Option B:** Lithium is not a first-line treatment for major depressive disorder. Although it may be used in some cases of treatment-resistant depression, its primary use is in the treatment of bipolar disorder.
**Option C:** Lithium is not used in the treatment of attention deficit hyperactivity disorder (ADHD). While it may have some effects on impulse control, its primary use is in the treatment of bipolar disorder.

**Clinical Pearl / High-Yield Fact**
Lithium has a narrow therapeutic index, and its serum levels must be carefully monitored to avoid toxicity, which can manifest as tremors, ataxia, and confusion.

**Correct Answer: D. Bipolar Disorder**