## **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 in this patient suggests a Müllerian duct anomaly, which could be part of a syndrome affecting the development of the female reproductive tract.

## **Why the Correct Answer is Right**
The next investigation of choice in a case of primary amenorrhea with a suspected Müllerian duct anomaly (like Mayer-Rokitansky-Küster-Hauser syndrome, MRKH) or other disorders of sexual development (DSD) is **Karyotyping (Option C)**. This is because the clinical presentation could also suggest disorders of sex development (DSD), where the genetic sex (XX, XY, or mosaic) may not match the phenotypic sex. Karyotyping helps in identifying genetic abnormalities such as Turner syndrome (45,X), Androgen Insensitivity Syndrome (46,XY), or other sex chromosomal anomalies.

## **Why Each Wrong Option is Incorrect**
- **Option A**: While an ultrasound can provide information on the presence or absence of the uterus and ovaries, it does not provide information on the genetic makeup of the patient, which is crucial in cases of primary amenorrhea with suspected DSD or chromosomal abnormalities.
- **Option B**: Hormonal assays can provide information on the levels of various hormones (like FSH, LH, estrogen) but do not directly address the possibility of a chromosomal abnormality.
- **Option D**: A pelvic MRI could offer detailed images of the pelvic anatomy, including the presence of a uterus, vagina, and ovaries, but similar to ultrasound, it does not provide genetic information.

## **Clinical Pearl / High-Yield Fact**
In cases of primary amenorrhea, it's crucial to consider a step-wise approach including history, physical examination, and investigations. A key point to remember is that when Müllerian duct anomalies are suspected, as indicated by the absence of the uterus and vagina, a **karyotype analysis** should be performed to rule out disorders of sex development (DSD) which could have implications for management, including potential fertility issues.

## **Correct Answer:** C. Karyotyping.