Okay, let's tackle this question. The patient is a 16-year-old girl with primary amenorrhea, a blind vaginal pouch, and absence of the uterus. So, the key here is to figure out the underlying cause.

Primary amenorrhea at 16 suggests either a problem with the reproductive tract development or hormonal issues. The blind vaginal pouch indicates a vaginal agenesis, which is often associated with Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome. In MRKH, there's absence of the uterus and upper two-thirds of the vagina, but the ovaries are usually normal. Since the patient has no uterus, the next step is to determine if the ovaries are functioning. That's where a karyotype comes in. A 46,XX karyotype would support MRKH, whereas a 45,X would suggest Turner syndrome, which is different.

The options for investigations might include a karyotype, pelvic ultrasound, hormonal tests, or imaging. The correct answer here is the karyotype because it confirms the chromosomal status. Pelvic ultrasound might not show a uterus, but the karyotype is needed to confirm the diagnosis. Hormonal tests could check for gonadal function, but the key is the chromosomal analysis. Other options like MRI might be used later but aren't first-line for this specific diagnosis.

So, the main point is that karyotype is essential to confirm the chromosomal status and differentiate MRKH from other conditions like Turner syndrome. The other options don't address the chromosomal aspect directly. The student should remember that in cases of vaginal agenesis with absent uterus, karyotype is the next step to establish the diagnosis.

**Core Concept**
This case highlights Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, a congenital disorder characterized by vaginal agenesis and uterine absence. The condition is chromosomally normal (46,XX), distinguishing it from Turner syndrome (45,X).

**Why the Correct Answer is Right**
Karyotype analysis confirms a 46,XX genotype, confirming MRKH syndrome. This is critical to differentiate from Turner syndrome (45,X), which presents with similar genital anomalies but chromosomal abnormalities. The absence of a uterus and a blind vaginal pouch in a 46,XX female points to Müllerian duct agenesis, the hallmark of MRKH.

**Why Each Wrong Option is Incorrect**
**Option A:** Pelvic ultrasound may show absence of a uterus but cannot confirm chromosomal status.
**Option B:** Hormonal assays (e.g., estradiol, FSH) assess ovarian function but do not diagnose MRKH.
**Option C:** MRI is useful for detailed anatomical imaging but is not first-line for diagnosing MRKH.
**Option D:** Genetic testing for Turner syndrome is unnecessary in a phenotypically female patient with MRKH.

**Clinical Pearl / High-Yield Fact**
MRKH syndrome affects 1 in 4,500 females and is associated with normal ovarian function and secondary sexual characteristics. Always order karyotype in vaginal agenesis with absent uterus to rule out Turner syndrome and confirm MRKH.

**Correct Answer: D. Karyotype**

✓ Correct Answer: B. Karyotyping