**Core Concept**
Transposition of the great arteries (TOF) is a congenital heart defect involving abnormal positioning of the great vessels, leading to mixed circulation. In the antenatal period, early diagnosis is crucial for planning management, and genetic evaluation plays a key role in identifying syndromic or inherited causes.

**Why the Correct Answer is Right**
Karyotyping is essential in the antenatal period to detect chromosomal abnormalities, such as trisomy 21 or other syndromic conditions, which may be associated with TOF. It helps determine whether the defect is isolated or part of a broader genetic syndrome, guiding prognosis and postnatal management. While surgical correction is definitive, antenatal karyotyping is a non-invasive diagnostic tool to assess genetic basis before birth.

**Why Each Wrong Option is Incorrect**
Option A: Balloon valvotomy is a postnatal intervention, not feasible or indicated in the antenatal period.
Option B: Open heart surgery is performed after birth and is not applicable during pregnancy.
Option D: Aspirin is used in specific maternal conditions like preeclampsia or antiphospholipid syndrome, not for TOF management.

**Clinical Pearl / High-Yield Fact**
TOF is often associated with chromosomal abnormalities; karyotyping should be performed in all cases of suspected congenital heart disease in utero to rule out syndromic forms. Early genetic diagnosis enables better counseling and preparation for postnatal care.

✓ Correct Answer: C. Karyotyping