**Core Concept**
The detection of fetal anomalies by antenatal ultrasound (USG) relies on the identification of specific morphological features or changes in fetal development. The earliest detection of fetal anomalies is crucial for timely intervention and management.

**Why the Correct Answer is Right**
The earliest detectable fetal anomaly by antenatal USG is typically the nuchal translucency (NT). The NT is the sonographic appearance of the subcutaneous tissue at the back of the fetal neck, which can be measured from 11 weeks of gestation. An increased NT thickness is associated with an increased risk of chromosomal abnormalities, such as Down syndrome. The measurement of NT is an essential component of the first-trimester screening for aneuploidy.

**Why Each Wrong Option is Incorrect**
**Option A:** The fetal cardiac activity, although visible on USG, is not typically the earliest detectable anomaly.
**Option B:** The fetal breathing movements, while visible on USG, are not as reliable or early a marker as the NT.
**Option C:** The fetal limbs and major body parts are not typically detectable until later in the first trimester.

**Clinical Pearl / High-Yield Fact**
The measurement of NT is a key component of the first-trimester screening for aneuploidy, and an increased NT thickness is associated with an increased risk of chromosomal abnormalities.

**Correct Answer:** C. Nuchal translucency.