Which of the following antipsychotics is not approved for MDD?

Risperidone is an atypical antipsychotic that is primarily used to treat conditions such as schizophrenia and bipolar disorder, and it is not routinely approved for major depressive disorder (MDD) as a standalone treatment. While there is some research on the use of antipsychotics in managing treatment-resistant depression, risperidone does not have the same level of approval specifically for MDD compared to some of the other listed medications.

In contrast, olanzapine has been studied in combination with other medications for the treatment of depressive episodes in bipolar disorder and can assist with depressive symptoms in treatment-resistant cases. Quetiapine is often used in the treatment of bipolar disorder and has been approved for major depressive episodes in the context of bipolar disorder. Brexpiprazole, while initially approved for schizophrenia, has also been indicated as an adjunctive therapy for MDD, showing efficacy in clinical trials.

Thus, the specific approval status related to MDD differentiates risperidone from the other antipsychotics mentioned, making it the best choice in response to the question.