**Core Concept**
Lithium is a mood stabilizer used in the treatment of bipolar disorder, particularly for managing acute mania. Its therapeutic levels are critical to prevent both efficacy and toxicity.

**Why the Correct Answer is Right**
Therapeutic levels of lithium are typically maintained between 0.6 and 1.2 meq/L. This narrow therapeutic index requires regular monitoring of serum lithium levels to avoid toxicity. Lithium exerts its effects by inhibiting the reuptake of serotonin and norepinephrine, thereby increasing their synaptic concentrations. This action is mediated by the inhibition of the sodium-potassium ATPase pump, which leads to an increase in intracellular sodium and a subsequent decrease in the activity of the enzyme inositol monophosphatase. This enzyme is crucial for the production of inositol, a second messenger involved in signal transduction pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.2 meq/L is below the therapeutic range, which would not be effective in managing acute mania.
**Option B:** 2 meq/L is above the toxic range, which could lead to severe lithium toxicity, including nephrotoxicity, neurotoxicity, and cardiac arrhythmias.
**Option C:** 0.4 meq/L is below the therapeutic range, which would not be sufficient to manage acute mania.

**Clinical Pearl / High-Yield Fact**
Lithium levels should be checked at least twice a week during the initial treatment phase and at least monthly thereafter to maintain therapeutic levels and prevent toxicity.

**Correct Answer:** C. 0.8 meq/L