What is the next step for a 17-year-old female who has never menstruated and is at Tanner stage III?

For a 17-year-old female who has never menstruated and is at Tanner stage III, performing a hormonal evaluation is the most appropriate next step. At Tanner stage III, one would typically expect that menstruation has begun, given that breast and pubic hair development are progressing. The absence of menstruation, or primary amenorrhea, at this age warrants assessment to understand the underlying cause.

A hormonal evaluation would help determine whether there are any abnormalities in the levels of estrogen, progesterone, and possibly other hormones that are necessary for normal menstrual function. Conditions such as hypothalamic amenorrhea, hypergonadotropic hypogonadism, or chromosomal abnormalities could contribute to this situation. By evaluating hormone levels, the clinician can gather critical information needed to direct further management or interventions.

Other potential steps, such as scheduling an MRI or initiating a pelvic exam, are not the immediate priorities without first understanding the hormonal status. Referring for genetic counseling could be relevant later if a genetic condition is suspected after initial evaluations, but it is not the immediate next step in this clinical scenario.