**Core Concept**
Early fetal anomaly detection using ultrasound (USG) is crucial for prenatal diagnosis and management. The earliest detectable anomalies are those with significant morphological changes, often involving the neural tube.

**Why the Correct Answer is Right**
The earliest fetal anomaly detectable by USG is the neural tube defect (NTD), specifically anencephaly. Anencephaly occurs due to the failure of the neural tube to close completely during embryonic development, resulting in the absence of a significant portion of the brain and skull. This defect is detectable as early as 10-14 weeks of gestation, making it the earliest fetal anomaly that can be identified by USG.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, but common early detectable anomalies not listed here include cardiac defects, which are detectable around 16-18 weeks.
**Option B:** This option is not specified, but other early detectable anomalies include limb abnormalities, detectable around 16-20 weeks.
**Option C:** This option is not specified, but other early detectable anomalies include gastrointestinal anomalies, detectable around 16-20 weeks.

**Clinical Pearl / High-Yield Fact**
Neural tube defects, including anencephaly, are often associated with a family history of similar defects. Early detection and counseling are crucial for parents to make informed decisions about their pregnancy.

**Correct Answer:** D.