A 35-year-old man with a 5-year diagnosis of OCD (contamination obsessions and washing compulsions) has completed 16 weeks of CBT with ERP and is on sertraline 200 mg daily for 4 months. His Y-BOCS score remains 28 (baseline 35). He reports 30% improvement in symptoms but continues to have significant functional impairment in his job and social relationships. He is compliant with treatment and motivated to improve. What is the most appropriate next step in management?