【单选题】 （2分）A 32-year-old woman comes to the office 8 months after her husband was killed in a motor vehicle collision. Since that time, she has had a decreased appetite and difficulty falling asleep. She states that she is often sad and cries frequently. She also has been rechecking the door lock five times before leaving her house and has to count exactly five pieces of toilet paper before she uses it. She says that she has always been a perfectionist but these urges and rituals are new. Medical history is unremarkable and she takes no medications. Pharmacotherapy should be targeted to which of the following neurotransmitters? （ ）