This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Normal Menstruation, Abnormal Menstruation, Menopausal Physiology and forsight of conception, Subject name: Gynaecology & Obstetrics
A 16-year-old female has not yet staed having menstrual cycles. Her secondary sex characteristics appear normal as does external examination of her genitalia. She is given an intramuscular injection of progesterone and 5 days later she has her first period. The mechanism responsible for the delay in menses is :- 
Select and explain. 
A. Hypoestronism, B. Hypopituitarism, C. An end-organ abnormality, D. A normal constitutional delay in menarche
D: A normal constitutional delay in menarche
Answer: D: A normal constitutional delay in menarche 
Explanation: The patient has primary amenorrhea. Her secondary sex characteristics are normal, indicating an adequate exposure to estrogen, hence ruling out hypoestrogenism and hypopituitarism. Although her external genitalia examination is normal, the proof of the lack of an end-organ abnormality ( such as Imperforate hymen / transverse vaginal septum/ unresponsive endometrium) is that she was able to bleed after being given progesterone. This also proves that her endometrial mucosa was primed with estrogen.