A 32-year-old man with a 5-year history of OCD presents for follow-up. He was started on sertraline 6 months ago and titrated to 150 mg/day. He reports 40% reduction in obsession severity and compulsion frequency, but still experiences significant distress and functional impairment. He attends weekly cognitive-behavioral therapy (CBT) sessions. His vital signs are normal, and he has no medical comorbidities. What is the most appropriate next management step?