An 11-year-old male is brought to the clinic by his parents for evaluation of contamination fears and excessive handwashing that have worsened over the past 6 months. He washes his hands 30-40 times daily, avoids touching doorknobs and shared surfaces at school, and has developed cracked and bleeding skin on his hands. His CY-BOCS score is 28, indicating severe OCD. He has missed 15 school days in the past 2 months due to his symptoms. He has been participating in weekly cognitive behavioral therapy with exposure and response prevention for the past 10 weeks with minimal improvement. He has no other psychiatric diagnoses and takes no medications. Which treatment plan is most appropriate?