A 13-year-old girl has a stable, nontender mass in the left groin, normal external female genitalia, and a karyotype of 46XY. What is the diagnosis?

The clinical presentation described is indicative of complete androgen insensitivity syndrome (CAIS). In this condition, individuals are genetically male (with a 46XY karyotype) but have external female genitalia due to the body's inability to respond to androgens, which are male hormones. This insensitivity results in the development of female-typical external anatomy, while internally, the individual may have undescended testes.

The presence of a stable, nontender mass in the groin likely corresponds to these undescended testes, which can remain located in the inguinal canal or can have an accessory position depending on the anatomical variations associated with CAIS. The individual would typically have normal female secondary sexual characteristics, if they went through puberty, because of the influence of estrogen from the conversion of testosterone in peripheral tissues.

Polycystic ovary syndrome, Turner syndrome, and congenital adrenal hyperplasia involve different pathophysiological mechanisms and presentations that do not align with the karyotype or the physical examination findings in this case. Polycystic ovary syndrome is related to ovarian function rather than chromosomal issues. Turner syndrome involves a 45X karyotype and typically presents with signs such as short stature and primary amenorrhea