An otherwise healthy 23-year old woman comes to the physician because she has not had a menstrual period for 2 months. Menarche occurred at the age of 14 years, and menses had occurred at regular 28-day intervals. Pelvic examination shows blue-tinged vaginal mucosa. The uterus is somewhat soft in the lower segment and is about the size of an orange. The adnexa are nontender with normal-sized ovaries. Which of the following is the most appropriate next step to establish the diagnosis?
A. Measurement of serum follicle-stimulating hormone concentration B. Measurement of serum thyroid-stimulating hormone concentration C. Measurement of urine B-human chorlonic gonadotropin concentration D. Ultrasonography of the pelvis E. Endometrial biopsy