## Core Concept
Maniac Depressive Psychosis (MDP), also known as Bipolar Disorder, is a mental health condition characterized by extreme mood swings that include emotional highs (mania or hypomania) and lows (depression). The treatment often involves mood stabilizers or antipsychotics.

## Why the Correct Answer is Right
Lithium is widely regarded as the first-line treatment for bipolar disorder due to its efficacy in stabilizing mood and preventing both manic and depressive episodes. It acts by modulating neurotransmitter activity and has a unique mechanism that is not fully understood but is believed to involve the inhibition of **inositol monophosphatase**, affecting **myo-inositol** levels in the brain, which plays a role in signal transduction.

## Why Each Wrong Option is Incorrect
- **Option A:** While certain antipsychotics can be used, especially in acute mania, they are not considered the first-line treatment for long-term management of MDP.
- **Option B:** Antidepressants can be used in the depressive phase but are not the drug of choice for MDP as a whole and can sometimes induce mania.
- **Option C:** Other mood stabilizers like valproate are effective but usually considered after lithium due to its side effect profile and long-term safety.

## Clinical Pearl / High-Yield Fact
A key point to remember is that lithium levels must be monitored regularly due to its narrow therapeutic index, with a target range typically between 0.6 and 1.2 mmol/L. Toxicity can occur at levels above 1.5 mmol/L.

## Correct Answer Line
**Correct Answer: D. Lithium.**