An 18-year old girl who has not attained menarche comes to casualty with acute retention of urine. The most likely diagnosis is:

One of the possible causes of 18 year old girl with primary amenorrhea is Imperforate Hymen which leads to hematocolpos and hematometra. This hematocolpos compresses the bladder neck against the symphysis pubic causing urinary retention. Urinary tract infection and cervical fibroid may also cause urinary retention, but would not present with primary amenorrhea.