A 39-year-old woman with bipolar I disorder is experiencing a persistent depressive episode despite eight weeks of adequate-dose lithium monotherapy with a serum level of 0.9 mEq/L. Her current depression has lasted four months, with a MADRS score of 32 indicating severe depression. She has no current psychotic features, no mixed features, no active suicidal ideation, and no rapid cycling pattern. Her last manic episode was two years ago, and she has never had an antidepressant-induced manic switch. Her lithium level, renal function, thyroid studies, and metabolic panel are all within normal limits. She has no history of metabolic syndrome. The PMHNP is planning the next step in treatment. Which of the following represents the most appropriate treatment planning approach?