Mr. Feisty has mild Alzheimer's disease (AD) diagnosed 2 years ago after progressive short-term memory decline. His wife reports that over the last three weeks he has shown irritable mood, anhedonia, early-morning awakening, low energy, reduced appetite with ~5 lb (2.3 kg) loss, and difficulty concentrating beyond his baseline cognitive impairment. He has skipped two community senior-center activities he previously enjoyed. No hallucinations or delusions. He denies active suicidal intent but says. "Sometimes I wonder if it would be easier if I didn't wake up." No recent illness, fever, or falls. No alcohol or illicit drug use. Which is a first-line medication option for treating depression in an elderly patient