A 32-year-old man presents with a two-year history of obsessive-compulsive disorder characterized by contamination obsessions and compulsive handwashing that consumes approximately four hours daily. His Yale-Brown Obsessive Compulsive Scale score is 28, indicating severe symptoms. He has been engaged in weekly exposure and response prevention therapy for 14 weeks with a trained therapist but reports only modest improvement. He has not yet tried any psychotropic medication. He is frustrated with his progress and asks whether medication should be added to his treatment plan. Which of the following best describes the appropriate treatment-planning rationale?