**Core Concept**
In the context of a first-trimester miscarriage, the investigation of choice is crucial for identifying potential underlying causes. ABO blood group incompatibility is a rare but significant cause of recurrent miscarriage, particularly in RhD-negative women carrying RhD-positive fetuses.

**Why the Correct Answer is Right**
The correct investigation of choice in this scenario involves assessing the ABO blood group and RhD status of both the mother and the father. This is because ABO blood group incompatibility can lead to the production of anti-A or anti-B antibodies, which can cause hemolysis of the fetus's red blood cells. RhD incompatibility, on the other hand, can lead to the production of anti-RhD antibodies, which can cause hemolysis of the fetus's red blood cells. Identifying these incompatibilities is essential for providing appropriate counseling and management.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasound is not the investigation of choice for recurrent miscarriage in the first trimester, although it may be used to confirm the diagnosis of miscarriage.
* **Option B:** Hysteroscopy is not typically used as a first-line investigation for recurrent miscarriage, although it may be used to evaluate the uterine cavity in cases of recurrent pregnancy loss.
* **Option D:** Karyotyping may be used to evaluate chromosomal abnormalities in the fetus, but it is not the investigation of choice for recurrent miscarriage in the first trimester.

**Clinical Pearl / High-Yield Fact**
ABO blood group incompatibility is a rare but significant cause of recurrent miscarriage, particularly in RhD-negative women carrying RhD-positive fetuses. All women experiencing recurrent miscarriage should be tested for ABO blood group and RhD status.

**Correct Answer: C. Karyotyping may be used to evaluate chromosomal abnormalities in the fetus, but it is not the investigation of choice for recurrent miscarriage in the first trimester.