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 :-

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.