**Question:** Drug specifically used for depressive phase of bipolar disorder is -

A. Lithium
B. Quetiapine
C. Fluoxetine
D. Olanzapine

**Core Concept:**
Bipolar disorder is a mood disorder characterized by extreme shifts in mood, energy, and behavior. It is classified into two phases: manic or hypomanic phase (elevated mood, increased energy, and reduced need for sleep) and depressive phase (low mood, decreased energy, and reduced motivation). Treatment strategies for bipolar disorder focus on managing both phases.

**Why the Correct Answer is Right:**
Lithium is a keystone medication for the management of bipolar disorder, particularly targeting the depressive phase. Lithium functions as a mood stabilizer by modulating ion channels, particularly sodium and potassium channels, and inhibiting the enzyme inositol monophosphatase, leading to decreased levels of inositol, which is essential for maintaining neuronal membrane integrity and preventing depolarization.

**Why Each Wrong Option is Incorrect:**

A. Quetiapine is an atypical antipsychotic drug primarily used for treating schizophrenia and schizoaffective disorder. Although it has some mood stabilizing effects, it is not as effective for bipolar depression as lithium.
B. Quetiapine is not specifically recommended for bipolar depression. It is primarily used for treating psychotic disorders.
C. Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) primarily used for treating depression and certain anxiety disorders. Fluoxetine is not a mood stabilizer and has limited efficacy in treating bipolar depression compared to lithium.
D. Olanzapine is an atypical antipsychotic drug primarily used for treating schizophrenia and bipolar disorder. While it has mood stabilizing effects, it is less effective for bipolar depression compared to lithium.

**Clinical Pearl:**
For a more comprehensive treatment of bipolar disorder, a combination of lithium or valproic acid (another mood stabilizer) with an antipsychotic drug (e.g., quetiapine or olanzapine) is often used. This combination therapy aims to target both manic and depressive phases of bipolar disorder.

**Correct Answer Explanation:**
Lithium is the drug of choice for the depressive phase of bipolar disorder. It is an effective mood stabilizer with well-established evidence supporting its use in bipolar depression. Lithium helps regulate neuronal excitability and modifies glutamate and GABA neurotransmission, which contributes to its mood stabilizing effect.

**Why Each Wrong Option is Incorrect:**

A. Quetiapine is a second-generation antipsychotic drug primarily used for treating schizophrenia and bipolar disorder. However, its mood stabilizing effects are less pronounced compared to lithium, making it less effective for bipolar depression.
B. Olanzapine is an atypical antipsychotic drug primarily used for treating schizophrenia and bipolar disorder. Though it has mood stabilizing effects, it is less effective in treating bipolar depression compared to lithium.
C. Fluoxetine is a selective