This is a classic pattern-recognition stem in reproductive endocrinology. Two discriminating features carry the diagnosis: breast development and sexual hair.
Because the breasts are well formed, oestrogen is acting on the body. Because the pubic and axillary hair are sparse, androgen action is failing - sexual hair is the most androgen-sensitive secondary sexual characteristic.
$\text{Oestrogenised body} + \text{androgen-unresponsive hair} \Rightarrow \text{Complete AIS}$
The mechanism: the patient is genetically $46,XY$ with intra-abdominal testes. The testes secrete anti-Müllerian hormone, which involutes the Müllerian structures, explaining the absent uterus. The testes also make testosterone, but a defective androgen receptor means target tissues ignore it; peripheral aromatisation of that testosterone to oestradiol then drives breast growth. The result is a phenotypically female individual with a short blind vagina, no uterus, and minimal sexual hair.
Contrast with MRKH ($46,XX$): there the uterus is also absent, but androgen receptors work, so pubic and axillary hair are entirely normal - which is not the case here.
\[\boxed{\text{Diagnosis} = \text{Complete Androgen Insensitivity Syndrome}}\]