Question

A 72-year-old woman is brought to the outpatient geriatric psychiatry clinic by her son due to a 6-month history of significant cognitive decline. He reports that she has become increasingly withdrawn, spends most of the day in bed, and has stopped managing her household finances. During the interview, she makes poor eye contact, exhibits psychomotor retardation, and frequently answers "I don't know" rather than attempting to answer orientation or memory questions. Her Montreal Cognitive Assessment (MoCA) score is 19/30. Medical history is significant for bipolar I disorder that was well-controlled on lithium for two decades; however, she discontinued all medications 14 months ago following the death of her husband. Her son notes she has not had any periods of elevated mood, decreased need for sleep, or rapid speech since stopping her medication. Vital signs and physical examination are normal. Laboratory studies, including complete blood count, comprehensive metabolic panel, thyroid-stimulating hormone, and vitamin B12 levels, are within normal limits. The physician determines that her cognitive deficits are secondary to a severe mood episode rather than a primary neurodegenerative process. Which of the following is the most appropriate initial pharmacological intervention for this patient?