Okay, let's tackle this question. The user wants me to create a detailed explanation for a multiple-choice question about differentiating between Mullerian agenesis and testicular feminization syndrome in primary amenorrhea. First, I need to recall what each condition entails.

Mullerian agenesis, also known as Mayer-Rokitansky-Küster-Hauser syndrome, is when the Mullerian ducts don't develop properly. This leads to the absence of the uterus and upper vagina, but the ovaries are usually normal. So, the hormones are working, but there's no organ to respond. On the other hand, testicular feminization syndrome, now called androgen insensitivity syndrome (AIS), is a genetic condition where the body doesn't respond to androgens. The person has testes but develops external female genitalia due to androgen insensitivity. However, since testes produce anti-Müllerian hormone (AMH), the Mullerian ducts regress, leading to absence of uterus and fallopian tubes. But in AIS, the karyotype is 46, XY, whereas in Mullerian agenesis, the karyotype is 46, XX.

The key difference is the karyotype. So, the investigation of choice would be karyotyping. Let me check the options. The correct answer should be karyotype analysis. The other options might include things like ultrasound, hormonal tests, or laparoscopy.

Wait, let's think about the other options. An ultrasound might show absence of uterus and ovaries in Mullerian agenesis, but in AIS, the testes might be present but in the abdomen. However, ultrasound can't determine karyotype. Hormonal tests like measuring AMH might help. In AIS, AMH is high because testes produce it, but in Mullerian agenesis, since the ovaries are present, AMH would be low. But maybe that's a stretch. Alternatively, HSG (hysterosalpingogram) could show absence of uterine structures, but again, can't differentiate between the two conditions based on that alone. So karyotype is the definitive test here.

Now, the user might have options like A. Ultrasound, B. Karyotype, C. Hormonal profile, D. Laparoscopy. So the correct answer is B. Karyotype. Let me structure the explanation accordingly. The core concept is the difference in karyotype between the two conditions. The correct answer is karyotype because it shows 46, XY in AIS and 46, XX in Mullerian agenesis. The wrong options are incorrect because other tests don't resolve the karyotype. The clinical pearl would be to remember that karyotype is key in differentiating these two causes of primary amenorrhea.

**Core Concept**
This question tests the ability to differentiate between two causes of primary amenorrhea—**Müllerian agenesis** (46,XX karyotype) and **testicular feminization syndrome** (46,XY karyotype)—using diagnostic investigations. The key distinction lies in **karyotype analysis**, which reveals chromosomal differences critical for diagnosis.

**Why the Correct Answer is Right**
**Karyotype analysis** is the definitive investigation.

✓ Correct Answer: C. Karyotype