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, sometimes until they bleed. She recognizes these thoughts are irrational but feels unable to resist the urge to wash. Her handwashing rituals consume 2–3 hours per day and significantly impair her work and social functioning. She denies depression, hallucinations, or suicidal ideation. What is the most appropriate first-line pharmacological intervention?