## Core Concept
Antipsychotic drugs primarily act by influencing neurotransmitter systems in the brain, particularly those involved in the regulation of mood, perception, and behavior. The main targets of antipsychotic drugs are dopamine and serotonin receptors.

## Why the Correct Answer is Right
The correct answer, **D. Dopamine D2 receptor antagonism**, is right because the primary mechanism of action of antipsychotic drugs involves the blockade of dopamine D2 receptors in the mesolimbic pathway of the brain. This blockade is thought to reduce the positive symptoms of psychosis, such as hallucinations and delusions, which are often associated with excessive dopaminergic activity. The dopamine hypothesis of schizophrenia suggests that an overactivity of dopamine pathways in the brain contributes to the symptoms of schizophrenia.

## Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because while antipsychotic drugs may have some effect on muscarinic receptors, this is not their primary mechanism of action. Some antipsychotics can cause anticholinergic side effects due to their muscarinic receptor antagonism, but this is not how they exert their antipsychotic effects.

**Option B:** This option is incorrect because, although modulation of the GABAergic system can influence psychiatric symptoms, antipsychotic drugs do not primarily act through GABA receptor modulation.

**Option C:** This option is incorrect because, while serotonin receptor antagonism (particularly 5-HT2A receptors) is a property of some antipsychotic drugs (atypical antipsychotics) and contributes to their therapeutic effects and side effect profiles, it is not the primary mechanism through which all antipsychotic drugs act.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the **potency of an antipsychotic drug is correlated with its affinity for the D2 receptor**. High-potency antipsychotics tend to have high affinity for D2 receptors and often produce more extrapyramidal side effects (EPS) due to their strong D2 blockade.

## Correct Answer: D. Dopamine D2 receptor antagonism