Androgen insensitivity is caused by a defective androgen receptor. DHT is responsible for creating male genitalia during fetal sexual development. The default human gender is female. So a genetically male patient with complete androgen insensitivity is externally phenotypically female. Lack of response to adrenal androgens prevents hair formation during puberty (adrenarche).

Just to add to explanation. Answer is not congenital adrenal hyperplasia, because pt has normal breast development and normal external genitalia. most common type 21-hydroxylase def would have precocious puberty or clitoral enlargement. Questions says patient has normal appearing external genitalia. 17a-hydroxylase def would lead to ambiguous genitalia in XY and in XX there would be no secondary sexual development because lack of sex steroids. But again, question says pt has normal breast development and external genitalia. In 11B-hydroxylase def, XX would have virilization, again not applicable here.

ectodermal dysplasia would have more characteristics like abnormal teeth, other abnormal findings from ectodermal layer.

a psych disorder could throw you off, maybe patient is not eating enough and is not having a period. But they said normal female body habitus, so I think that seems less likely compared to androgen insensitivity.

Sex chromosome mosaicism made me think of Turner Syndrome. There is a unique body habitus for Turner Syndrome so makes this a wrong answer.