This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'What is the optimal time during the menstrual cycle when serum progesterone should be drawn to confirm the diagnosis of luteal phase deficiency : \nSelect and explain. \nA. Day 18, B. Day 21, C. Day 23, D. Day 25', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Day 25', 'role': 'assistant'}]