**Core Concept**
Lithium is a mood stabilizer used in the treatment of bipolar disorder. It has a narrow therapeutic index, requiring careful dosing to achieve therapeutic effects while minimizing toxicity.

**Why the Correct Answer is Right**
The therapeutic dose of lithium is typically around 900-1200 mg/day, which corresponds to a plasma concentration of 0.8-1.2 mEq/L. This range is effective in managing symptoms of bipolar disorder, such as mania and depression. Lithium's mechanism of action is thought to involve inhibition of the enzyme inositol monophosphatase, leading to decreased glycogen synthesis and increased serotonin and dopamine levels in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely too low, as lithium doses below 600 mg/day may not achieve sufficient therapeutic effects.
**Option B:** This option is likely too high, as lithium doses above 1800 mg/day can increase the risk of toxicity, including nephrotoxicity and neurotoxicity.
**Option C:** This option is not specific enough, as the therapeutic dose of lithium can vary depending on the individual patient and clinical context.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor lithium levels regularly in patients taking this medication, as toxicity can occur at doses only slightly above the therapeutic range.

**Correct Answer: C. 900-1200 mg/day**