Which medication is NOT an antipsychotic approved for treatment-resistant Major Depressive Disorder (MDD)?

The correct choice is the medication that is not an antipsychotic approved for treatment-resistant Major Depressive Disorder (MDD). Amitriptyline is a tricyclic antidepressant and does not fall under the category of antipsychotic medications. It works by increasing the levels of neurotransmitters in the brain that are thought to be related to mood regulation, and it is primarily used for treating depression and certain anxiety disorders, but not specifically as an antipsychotic.

In contrast, other options provided are antipsychotic medications that have been investigated or approved for treating treatment-resistant depression. Abilify (aripiprazole) and Seroquel XR (quetiapine) are atypical antipsychotics often used as adjuncts in treating severe depression. Symbyax, a combination of fluoxetine and olanzapine, is specifically approved for treatment-resistant depression as well. Hence, the only medication listed that is not classified as an antipsychotic is Amitriptyline, making it the correct answer.