A 35-year-old man with a 6-year history of OCD presents to the outpatient clinic. He has been on sertraline 150 mg daily for 8 weeks with minimal improvement in his contamination obsessions and compulsive washing (currently spending 3–4 hours per day washing). His Yale-Brown Obsessive Compulsive Scale (Y-BOCS) score is 32 (severe). He reports good adherence and tolerates the medication well. His wife reports that his symptoms are severely impairing his work and family functioning. What is the most appropriate next step?