## Core Concept
Primary amenorrhea refers to the absence of menstruation in a woman by the age of 16. The presence of a short blind vagina and absent uterus suggests a Müllerian duct anomaly, which is a congenital disorder affecting the development of the female reproductive tract.

## Why the Correct Answer is Right
The next investigation of choice in this scenario would be to determine the presence or absence of ovaries and to evaluate for any associated genetic abnormalities. This is because the Müllerian duct anomalies can be part of a syndrome that affects ovarian development or function. **Karyotyping (Option C)** is the correct answer because it helps identify genetic conditions such as Turner syndrome (45,X), Androgen Insensitivity Syndrome (46,XY), or other sex chromosomal anomalies that could be associated with primary amenorrhea and Müllerian duct anomalies.

## Why Each Wrong Option is Incorrect
- **Option A**: While an ultrasound can provide information about the presence of ovaries and the anatomy of the reproductive tract, it does not provide information on the genetic makeup of the individual.
- **Option B**: Hormonal assays can provide information on the functional status of the ovaries but do not directly address the underlying cause of the absent uterus and vagina.
- **Option D**: A pelvic MRI could provide detailed anatomical information but is not the first line in investigating the genetic basis of the condition.

## Clinical Pearl / High-Yield Fact
A key point to remember is that in cases of primary amenorrhea with Müllerian duct anomalies, **the presence of testes (as in Androgen Insensitivity Syndrome) or a Y chromosome** significantly influences management due to the risk of gonadal malignancy. Therefore, karyotyping is crucial.

## Correct Answer: C. Karyotyping.