## Core Concept
Lithium is a medication primarily used in the treatment of **bipolar disorder** and has several other applications. It is a mood stabilizer with a narrow therapeutic index. Understanding its indications is crucial for its effective and safe use.

## Why the Correct Answer is Right
Lithium is commonly indicated for the treatment of **bipolar disorder**, particularly for managing manic episodes and for long-term mood stabilization. It is also used in certain cases of **unipolar depression** (as an adjunct to antidepressants) and for the treatment of **schizoaffective disorder**. However, its use in **myasthenia gravis** is not a recognized indication. In fact, lithium can potentially worsen myasthenia gravis symptoms.

## Why Each Wrong Option is Incorrect
- **Option A:** Not provided, assuming relates to a recognized use of lithium.
- **Option B:** Not provided, assuming relates to a recognized use of lithium.
- **Option C:** Not provided, assuming relates to a recognized use of lithium.
- **Option D: Myasthenia Gravis** - This condition is not an indication for lithium. Lithium can actually exacerbate muscle weakness.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that lithium levels must be monitored closely due to its narrow therapeutic index. Levels are typically maintained between 0.6 and 1.2 mmol/L. Toxicity can occur at levels above 1.5 mmol/L.

## Correct Answer: D. Myasthenia Gravis