**Core Concept:** Lithium is a mood-stabilizing medication used in the treatment of bipolar disorder. It works by modulating ion channel activity and protein kinase C activity in the brain. Therapeutic levels of lithium are essential to ensure efficacy and minimize side effects.

**Why the Correct Answer is Right:** The correct answer is **D** as it represents the therapeutic range of lithium levels in serum, which is typically within 0.6-1.2 mEq/L (millimole per liter). Maintaining lithium levels within this range is crucial for optimal treatment of bipolar disorder. Lower levels may be insufficient for mood stabilization, while higher levels can lead to toxicity and side effects.

**Why Each Wrong Option is Incorrect:**

A. 0.4-0.8 mEq/L - This range is too low and may not be effective in treating bipolar disorder.
B. >1.2 mEq/L - This range is too high and can cause lithium toxicity and severe side effects.
C. <0.4 mEq/L - This range is too low and may not be effective in treating bipolar disorder. **Why Lithium is Used:** Lithium has been used for decades in the treatment of bipolar disorder due to its effectiveness and proven efficacy. It helps in stabilizing mood, reducing the frequency and severity of manic and depressive episodes, and improving overall functioning in patients with bipolar disorder. **Clinical Pearl:** Regular monitoring of lithium levels is essential to ensure optimal therapeutic response and minimize adverse effects. A simple mnemonic to remember the therapeutic range is "HILO" (High, Intermediate, Low, Optimal). This can help guide clinicians in adjusting lithium dose based on serum level and clinical response. **Correct Answer:** D. 0.6-1.2 mEq/L