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
What is the optimal time during the menstrual cycle when serum progesterone should be drawn to confirm the diagnosis of luteal phase deficiency : 
Select and explain. 
A. Day 18, B. Day 21, C. Day 23, D. Day 25
D: Day 25
Answer: D: Day 25 
Explanation: Day 25