Review Question - QID 109345

A 16-year-old girl presents to the physician for delayed onset of menstruation. She reports that all of her friends have experienced their 1st menses and she wonders whether “something is wrong with me.” The patient is a sophomore in high school and doing well in school. Her past medical history is significant for an episode of streptococcal pharyngitis 6 months ago for which she was treated with oral amoxicillin. The patient is in the 35th percentile for weight and 5th percentile for height. On physical exam, her temperature is 98.7°F (37.1°C), blood pressure is 112/67 mmHg, pulse is 71/min, and respirations are 12/min. The patient has a short neck and wide torso. She has Tanner stage 1 breast development and pubic hair with normal external female genitalia. On a bimanual exam, the vagina is of normal length and the cervix is palpable. Which of the following is the most accurate test to diagnose this condition?