A 32-year-old man with a 5-year history of OCD (contamination obsessions and checking compulsions) has been on sertraline 200 mg daily for 8 weeks with minimal improvement in symptoms. Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) score remains 28 (severe). He is compliant with medication and denies comorbid depression or substance use. What is the most evidence-based next step in management?