A 22 years old woman presents with complaints of intense urge to shower. She spends hours showering and finds it difficult to get out of shower. She is also following same order of actions while bathing, scrubbing every body part 3 times in the same order. She is aware that this is abnormal but she becomes very anxious if she tries to break this pattern. She works as receptionist and is frequently late to her job because of this habit. She gets along well with friends at work. The drug of choice for treating her condition is: