Okay, let's tackle this question. The patient is a 16-year-old female with primary amenorrhea. Primary amenorrhea means she hasn't had her first period by age 16. The exam shows a short blind vagina and an absent uterus. So, the first thing that comes to mind is a structural abnormality. The question is asking for the next investigation of choice.

Primary amenorrhea can have various causes, including genetic, hormonal, or anatomical issues. Since there's a short blind vagina and no uterus, this points towards a developmental issue. The most likely condition here is Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome. MRKH is characterized by the absence of the uterus and upper two-thirds of the vagina, with normal external genitalia and secondary sexual characteristics. The karyotype is usually 46,XX.

Now, the next step in investigation would be to determine the cause. Since the patient has a blind vagina and absent uterus, imaging would be the next step. Ultrasound might not be sufficient because the uterus is absent. MRI is more detailed and can confirm the absence of the uterus and upper vagina. Additionally, checking for other associated anomalies like renal agenesis or skeletal abnormalities could be part of the workup, but the primary next step here is imaging.

Looking at the options, if the correct answer is MRI, then the other options might be things like karyotype analysis, hysterosalpingogram, or pelvic ultrasound. Karyotype would be useful if there's a suspicion of Turner syndrome (45,X), but in MRKH, the karyotype is normal. Pelvic ultrasound might not show the uterus if it's absent, so MRI is better. Hysterosalpingogram requires a patent vagina and uterus, which she doesn't have. So the next best step is MRI.

**Core Concept**
Primary amenorrhea with a short blind vagina and absent uterus suggests **Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome**, a congenital disorder of Müllerian duct development. The diagnosis requires **imaging** to confirm uterine absence and assess associated anomalies.

**Why the Correct Answer is Right**
The next investigation of choice is **pelvic MRI**. It provides high-resolution imaging to confirm the absence of the uterus and upper vagina, while also evaluating for associated abnormalities (e.g., renal agenesis, skeletal anomalies). MRKH patients have a 46,XX karyotype and normal ovaries, so MRI helps distinguish it from other causes like androgen insensitivity or Turner syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Pelvic ultrasound may miss subtle Müllerian anomalies and cannot reliably detect a blind vagina.
**Option B:** Karyotype analysis is unnecessary here (MRKH has a normal 46,XX karyotype) and is reserved for cases with ambiguous genitalia or Turner-like features.
**Option C:** Hysterosalpingogram requires a patent vagina and functional uterus, which are absent in this case.

**Clinical Pearl**
MRKH is the most common cause of **congenital absence of the uterus and upper vagina** in phenotypic females. Always order **MRI** to confirm the diagnosis and screen for renal/skeletal abnormalities.

**Correct Answer: C. Pelvic MRI**

✓ Correct Answer: A. Karyotyping