Review Question - QID 214356

A 31-year-old G2P1001 woman presents to the labor and delivery floor triage at 38 weeks of gestation because she has been experiencing regular contractions for the last 3 hours. She says that the contractions have been increasing in frequency and have become increasingly uncomfortable over time. On presentation, she is found to be 2 cm dilated with ruptured membranes so she is kept for observation. Over the next 14 hours, she slowly dilates to 4 cm so the decision is made to start a hormone to enhance contractions. Which of the following is the endogenous site of production for this hormone?