**Core Concept**
The underlying principle being tested is the diagnosis of primary amenorrhea with specific focus on **Mullerian agenesis** and **Disorders of Sex Development (DSD)**. Primary amenorrhea is defined as the failure to menstruate by the age of 16 years in the presence of normal secondary sexual characteristics or by the age of 14 if there are no secondary sexual characteristics.

**Why the Correct Answer is Right**
Given the clinical presentation of normal breast development, sparse axillary and pubic hair, a normal vagina, and the absence of the uterus on ultrasound, combined with the presence of solid nodular structures in the bilateral inguinal regions, the diagnosis points towards **Androgen Insensitivity Syndrome (AIS)**. In AIS, individuals have a **46,XY karyotype**, testes (which may be undescended and found in the inguinal canals), and a resistance to androgen action, leading to the development of female external genitalia despite the presence of testes.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not align with the provided clinical scenario.
**Option B:** Incorrect as it does not match the given description.
**Option C:** Incorrect because it is not consistent with the clinical findings.
**Option D:** Incorrect as the description does not fit the clinical presentation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that individuals with **Androgen Insensitivity Syndrome** often present with primary amenorrhea, normal female external genitalia, and the absence of the uterus, with testes located in the inguinal canals or abdomen. The presence of testes in unusual locations is a critical diagnostic clue.

**Correct Answer:** D. Androgen Insensitivity Syndrome.