A 17-year-old girl is brought to the clinic by her mother, who says that she has become “increasingly difficult to get along with” since the academic year began 5 months ago. Her daughter is irritable and is having difficulty concentrating on her schoolwork, resulting in some poor grades. The mother has found the patient, who says that she is always tired, tossing in her sleep at night. She recently developed acne and is concerned that her boyfriend might reject her. As a result, she spends lots of time worrying about her appearance and applying makeup to conceal her acne. The patient has always been considered by friends and family to be “high strung.” She admits to feeling extremely apprehensive when taking tests at school and “on edge” at her part-time job at a local restaurant. She believes this stems from her naturally competitive nature and her desire to be valedictorian, with the pressure worsening this school year as graduation approaches. The patient obsesses about being accepted at a top university and how her family will pay for monthly expenses in addition to her education. She feels powerless to control her thoughts and wishes that she could change. Which of the following is the most likely diagnosis?