A 38-year-old woman presents to the psychiatry outpatient clinic with a 3-month history of persistent low mood, anhedonia, and poor concentration. She reports waking up at 4 AM daily and being unable to fall back asleep. Her appetite has decreased significantly, and she has lost 6 kg unintentionally. She denies suicidal ideation but admits to feeling hopeless about the future. Her past medical history includes hypothyroidism (on levothyroxine, TSH normal). Mental status examination reveals psychomotor retardation, depressed affect, and impoverished speech. She has no history of manic or hypomanic episodes. What is the most appropriate first-line pharmacological intervention for this patient?