## **Core Concept**
Anencephaly is a severe congenital anomaly characterized by the absence of a major portion of the brain, skull, and scalp. It results from the failure of neural tube closure during embryonic development. Early diagnosis is crucial for family counseling and pregnancy management.

## **Why the Correct Answer is Right**
The correct answer, **10 weeks**, is based on the timing of when anencephaly can be reliably detected using ultrasound technology. Anencephaly is typically diagnosed between 10 to 14 weeks of gestation. This early detection is possible because the anomaly involves a significant structural defect that can be visualized with transvaginal or transabdominal ultrasound.

## **Why Each Wrong Option is Incorrect**
- **Option A: 6 weeks** - At 6 weeks, the embryo is still very small, and detailed structural anomalies like anencephaly might not be clearly visible even with high-resolution ultrasound. Detection at this stage is generally not reliable for anencephaly.
- **Option B: 16 weeks** - Although anencephaly can be diagnosed at 16 weeks, waiting until this gestational age may not be necessary given that reliable detection can occur earlier.
- **Option D: 20 weeks** - By 20 weeks, anencephaly is easily detectable, but this is later than the earliest possible diagnosis, which can be made around 10 weeks.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that early detection of anencephaly allows for timely counseling and decision-making regarding pregnancy management. Ultrasound markers, including the absence of the cranial vault and the presence of the "frog-like" appearance of the fetal head, are critical for the diagnosis.

## **Correct Answer: B. 10 weeks**