**Core Concept**
Androgen insensitivity syndrome (AIS) is a genetic disorder affecting individuals with XY chromosomes, where the body is unable to respond to androgens (male hormones), resulting in the development of female external genitalia despite the presence of testes. This condition is characterized by primary amenorrhea, normal breast development, and absence of pubic hair due to inadequate androgen stimulation.

**Why the Correct Answer is Right**
The patient's presentation of primary amenorrhea, normal breast development, and absence of pubic hair, along with the USG findings of absent uterus, is consistent with AIS. In this condition, the presence of testes produces androgens, but the body's inability to respond to these hormones leads to the development of female external genitalia. The absence of a uterus is due to the lack of Müllerian duct development, which occurs under the influence of anti-Müllerian hormone produced by the testes. The patient's bilateral inguinal swelling is likely due to undescended testes, which are characteristic of AIS.

**Why Each Wrong Option is Incorrect**
**Option B:** Turner syndrome is characterized by short stature, ovarian dysgenesis, and the presence of a 45,X karyotype. It does not typically present with primary amenorrhea and absent uterus in the absence of a 45,X karyotype.

**Option C:** Müllerian agenesis, also known as Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, is characterized by the absence of the uterus and upper part of the vagina, but the ovaries and external genitalia are typically normal. The patient's presentation of absent uterus and normal breast development, along with the absence of pubic hair, makes this diagnosis less likely.

**Option D:** Klinefelter syndrome is characterized by the presence of XXY chromosomes, testicular dysgenesis, and the presence of a small, firm testis. It typically presents with gynecomastia, infertility, and a tall stature, which does not match the patient's presentation.

**Clinical Pearl / High-Yield Fact**
AIS is a critical diagnosis to consider in patients with primary amenorrhea and normal breast development, as it requires a coordinated approach to management, including the administration of estrogen for feminization and the removal of the testes to prevent malignancy.

**✓ Correct Answer: A. Androgen insensitivity syndrome**