**Core Concept**
Lithium (Li) is a mood stabilizer used in the treatment of bipolar disorder. Its mechanism of action is complex and multifaceted, involving the inhibition of glycogen synthase kinase-3 (GSK-3), a key enzyme in the signaling pathways of various neurotransmitters.

**Why the Correct Answer is Right**
Lithium's primary mechanism of action is the inhibition of GSK-3, which leads to an increase in the activity of certain neurotransmitters such as serotonin and norepinephrine. This results in improved mood stability and reduced symptoms of mania. Lithium also has neuroprotective effects, which may contribute to its therapeutic efficacy. Its ability to modulate the activity of GSK-3 makes it a valuable treatment option for bipolar disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because lithium is not primarily used as an antipsychotic medication. While it may be used in conjunction with antipsychotics, its primary mechanism of action is as a mood stabilizer.
**Option B:** This option is incorrect because lithium is not a first-line treatment for major depressive disorder. While it may be used as an adjunctive therapy, its primary use is in the treatment of bipolar disorder.
**Option C:** This option is incorrect because lithium is not primarily used as an anxiolytic medication. While it may have some anxiolytic effects, its primary mechanism of action is as a mood stabilizer.

**Clinical Pearl / High-Yield Fact**
Lithium's narrow therapeutic index means that serum levels must be carefully monitored to avoid toxicity, which can cause symptoms such as tremors, ataxia, and confusion.

**Correct Answer:** B.