**Core Concept**
The earliest detectable congenital malformation by ultrasound (USG) refers to the earliest stage at which an abnormality in fetal development can be visualized using this imaging technique. This concept is crucial in prenatal diagnosis and monitoring of fetal growth and development.

**Why the Correct Answer is Right**
The earliest detectable congenital malformation by USG is typically a neural tube defect (NTD), such as anencephaly or spina bifida. This is because the neural tube forms early in embryonic development, around 23-26 days post-fertilization, and any abnormalities in its closure can be detected by USG as early as 5-6 weeks of gestation. The detection of NTDs is possible due to the visualization of the absence or abnormal closure of the cranial vault or spinal cord.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while cardiac anomalies can be detected by USG, they are not the earliest detectable congenital malformation.
**Option B:** This option is incorrect because chromosomal abnormalities, such as Down syndrome, can be detected by USG, but they are not a type of structural congenital malformation.
**Option C:** This option is incorrect because while abdominal wall defects, such as omphalocele, can be detected by USG, they are not the earliest detectable congenital malformation.

**Clinical Pearl / High-Yield Fact**
The neural tube is a critical structure in fetal development, and any abnormalities in its closure can have significant implications for fetal health and development. It is essential for obstetricians and sonographers to be aware of the earliest detectable congenital malformations by USG to provide accurate prenatal diagnosis and counseling.

**Correct Answer: C. Spina Bifida.**