We present a case of a 13-year-old Filipino patient with ambiguous genitalia, randomly assigned as female at birth and was subsequently reared as such. Starting at 10 years of age, secondary male sexual characteristics emerged. However, medical consultation was sought only when the patient developed a gradually enlarging and painful abdominal mass. After cytogenetic studies, we are able to identify the case as Disorder of Sex Development, 45,X/46,X,der(Y) mosaicism, mixed gonadal dysgenesis. Further imaging, laparotomy and histopathology, revealed that the abdomino-pelvic mass was from the right ovary, composed of immature teratoma. Ductus deferens and epididymis were also identified within the right ovary. At the time of presentation, the patient was already experiencing gender dysphoria. This report not only highlights the wide spectrum of phenotypes in mixed gonadal dysgenesis, but also stresses the importance of proper gender assignment during the newborn period in patients with disorders of sex development.

Key Words: disorder of sexual development, ambiguous genitalia, mixed gonadal dysgenesis, mosaicism, 45,X/46,X,der(Y) karyotype, immature teratoma, gender dysphoria