Lithium: A. has a very narrow therapeutic range. B. completely crosses into the brain. C. crosses the blood-brain barrier rapidly. D. is metabolized in the liver into two active metabolites. Solution Answer: A. has a very narrow therapeutic range. Lithium is potent mood stabilizer but has very narrow therapeutic range because therapeutic dose is very nearer to toxic dose. Therefore, close monitoring is essential when a patient is prescribed with this drug. Lithium cannot cross blood-brain barrier easily.