A 19-year-old man working as a driver comes to Psychiatrist with excessive anxiety and fear. He repos that every time he drives over a bump in the road, he is convinced that he has accidentally run over a small child. He has to pull over and check underneath his car for blood and retrace his driving route to look for any injured children. As a result, he is always late for work.He also has intrusive thoughts about stabbing his coworkers.He prays to try to erase these thoughts from his mind, but this rarely helps. First-line pharmacological treatment of this patient's condition primarily affects which of the following neurotransmitters?