This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Reproductive physiology and hormones in females, Subject name: Gynaecology & Obstetrics
[{'content': 'Subnuclear vacuolation is a characteristic sign of \nSelect and explain. \nA. Proliferative phase of endometrium, B. Secretory phase of endometrium, C. Menstruation, D. Regeneration of endometrium', 'role': 'user'}, {'content': "Answer: B \nExplanation: The most characteristic signs of secretory phase are found in the glands. Their epithelial cells develop spherical translucent areas between the nuclei and the basement membrane which contain the precursors of the glandular secretion and which persist until about the 21st day of the cycle. This characteristic appearance is called subnuclear vacuolation and is presumptive evidence of progesterone activity and therefore of ovulation.Reference: Shaw's Textbook of Gynecology; 16th edition; Chapter 2; Normal Histology", 'role': 'assistant'}]