A 17-year-old female at Tanner stage III of sexual development has never had her menses. What is the most likely diagnosis?

A 17-year-old female at Tanner stage III who has never had her menses is most likely experiencing primary amenorrhea. Primary amenorrhea is defined as the absence of menstruation by the age of 15 in females who have developed secondary sexual characteristics, or by the age of 16 in females without secondary sexual development. Since this individual has reached Tanner stage III, which indicates some level of sexual maturation, it highlights that she has not yet started menstruating.

Given her age and development stage, the diagnosis of ovarian dysgenesis could be a consideration; however, this condition usually presents with additional symptoms or traits such as short stature or a webbed neck, which are not mentioned in the scenario.

Dysmenorrhea refers to painful menstruation and would not apply here since the patient has never had a menstrual cycle. On the other hand, secondary amenorrhea is defined as the cessation of menstruation after it has been established for a period of time; hence it does not apply to this scenario where menstruation has never started.

Thus, the most appropriate diagnosis for the described case is indeed primary amenorrhea, given the absence of menstruation in the context of her sexual development stage.