**Core Concept:** Lithium is a mood-stabilizing agent used in the treatment of bipolar disorder. It acts as a serotonin-norepinephrine-dopamine reuptake inhibitor and has an impact on multiple neurotransmitter systems. Lithium toxicity occurs when serum concentration exceeds a certain threshold.

**Why the Correct Answer is Right:** The correct answer is **D:** 0.8-1.2 mEq/L. Lithium toxicity occurs when the serum concentration exceeds this range. At this level, it interferes with the normal renal excretion of lithium and the cumulative amount in the body increases, leading to toxicity.

**Why Each Wrong Option is Incorrect:**

A. **0.6-0.7 mEq/L:** This is within the therapeutic range and does not indicate lithium toxicity.

B. **1.2-1.4 mEq/L:** This is slightly outside the therapeutic range but still falls within the range of safety.

C. **1.4-1.5 mEq/L:** This is still within the therapeutic range and does not indicate lithium toxicity.

**Clinical Pearl:** A serum lithium concentration of 0.8-1.2 mEq/L is considered therapeutic and helps in the maintenance of bipolar disorder treatment. Monitoring serum lithium levels is crucial to ensure optimal treatment and prevent toxicity.

**Why Lithium Toxicity is Dangerous:** Lithium toxicity can cause multiple organ system involvement, including cardiac, renal, and neurological effects. The most common clinical features are neuropsychiatric symptoms (e.g., tremors, ataxia, confusion, and psychosis). Lithium toxicity can also lead to renal dysfunction and hyperkalemia. Serum lithium concentration is a crucial tool in monitoring lithium therapy and preventing toxicity.