**Core Concept**
The combination of multiple antipsychotics can lead to increased risk of adverse effects due to additive or synergistic effects on various neurotransmitter systems, particularly dopamine and serotonin pathways.

**Why the Correct Answer is Right**
When two or more antipsychotics are used concurrently, the risk of **extrapyramidal side effects (EPS)**, such as dystonia, parkinsonism, and tardive dyskinesia, increases due to the cumulative blockade of dopamine D2 receptors in the striatum. This is because antipsychotics primarily work by blocking dopamine receptors, and combining multiple agents can lead to excessive blockade, causing these side effects. Additionally, the combination of antipsychotics can also increase the risk of **hyperprolactinemia** due to the blockade of dopamine receptors in the pituitary gland, which normally inhibits prolactin secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** **Weight gain** is a common side effect of antipsychotics, and combining multiple agents can exacerbate this effect due to the increased blockade of dopamine receptors, which can lead to increased appetite and food intake.

**Option B:** **Increased risk of seizures** is not a direct consequence of combining multiple antipsychotics. However, some antipsychotics, such as clozapine, may increase the risk of seizures, but this is not a characteristic of all antipsychotics.

**Option C:** **Increased risk of QT interval prolongation** is a concern when combining antipsychotics that have a high risk of QT interval prolongation, such as ziprasidone and risperidone. This can increase the risk of torsades de pointes and other arrhythmias.

**Clinical Pearl / High-Yield Fact**
When combining antipsychotics, it's essential to monitor for signs of EPS, such as dystonic reactions, and to adjust the medication regimen accordingly. Additionally, patients on combined antipsychotic therapy should be regularly assessed for weight gain and other metabolic side effects.

**Correct Answer:** D.