Which of the following is an example of a mood stabilizer used in bipolar disorder?

Lithium is a classic mood stabilizer that is widely used in the treatment of bipolar disorder. It is effective in both the acute management of manic and hypomanic episodes as well as in the long-term stabilization of mood to prevent future episodes. Lithium's mechanism of action involves modulation of neurotransmitter release and neuroprotective effects, which contribute to its efficacy in stabilizing mood fluctuations commonly seen in bipolar disorder.

In contrast, the other options listed are more specific to other classes of medications. Fluoxetine and sertraline are selective serotonin reuptake inhibitors (SSRIs) that are primarily used as antidepressants and may not be first-line treatments for bipolar disorder due to the risk of triggering manic episodes. Quetiapine is an atypical antipsychotic that can be used in the treatment of bipolar disorder, especially for acute mania or depression, but it is not classified primarily as a mood stabilizer. Lithium remains the archetypal mood stabilizer recognized specifically for its role in managing bipolar disorder.