at BMI 15 not only has she never had a period but she never had a meal.
Androgen Insensitivity Syndrome - Defect in androgen receptor resulting in normal-appearing female (46,XY DSD). Functioning testes causes increased testosterone at puberty, which is converted to estrogen peripherally, giving female secondary sexual characteristics (female external genitalia). Lack of androgen receptor function leads to absent or scant axillary and pubic hair. Patients have rudimentary vagina, but uterus and fallopian tubes absent.
Androgen insensitivity syndrome is the answer but you might have considered Müllerian agenesis (Mayer-Rokitansky- Küster-Hauser syndrome).
Mullerian agenesis will have normal hormone levels and may present as 1° amenorrhea (due to a lack of uterine development) in females with fully developed 2° sexual characteristics (functional ovaries). Hair development is normal as well. Patients also have normal height.
Seems like this question did not give us much to distinguish besides height and tanner stage 1 pubic/axillary hair.
Androgen insensitivity syndrome
Defect in androgen receptor resulting in normal-appearing female (46,XY DSD); female external genitalia with scant axillary and pubic hair, rudimentary vagina; uterus and fallopian tubes absent due to persistence of anti-Müllerian hormone from testes.
Patients develop normal functioning testes (often found in labia majora; surgically removed to prevent malignancy). High testosterone, estrogen, LH (vs sex chromosome disorders).
Paramesonephric (Müllerian) duct develops into female internal structures— fallopian tubes, uterus, upper portion of vagina (lower portion from urogenital sinus). Male remnant is appendix testis.
Müllerian agenesis (Mayer-Rokitansky- Küster-Hauser syndrome)—may present as 1° amenorrhea (due to a lack of uterine development) in females with fully developed 2° sexual characteristics (functional ovaries).