Final answer:
The best adjunctive agent for the treatment of acute mania with lithium is antipsychotic medications.
Step-by-step explanation:
The best adjunctive agent for the treatment of acute mania with lithium is antipsychotic medications. These medications are often used in combination with lithium to help control symptoms of mania, such as agitation, hyperactivity, and delusions. Common antipsychotics used as adjunctive therapy include risperidone, olanzapine, and quetiapine. These medications work by blocking certain chemicals in the brain that are thought to contribute to the symptoms of mania.