A 28-year-old woman presents to the psychiatry clinic with a 3-year history of intrusive thoughts that her hands are contaminated with harmful bacteria, despite frequent handwashing. She washes her hands 15–20 times daily, often until they bleed, yet the anxiety persists. She recognizes these thoughts as irrational but feels unable to resist the urge to wash. Her work performance has declined, and she avoids social gatherings. She denies low mood, anhedonia, or suicidal ideation. What is the most appropriate first-line pharmacological intervention?