**Core Concept**
The diagnosis of anencephaly, a congenital anomaly where a large part of the brain and skull are absent, is crucial for prenatal counseling and planning. Accurate ultrasound detection is essential for early identification and management. The normal development of the fetal brain and skull is critical for this diagnosis.

**Why the Correct Answer is Right**
Anencephaly is a severe congenital anomaly that can be detected on ultrasound by the absence of the cranial vault and the presence of a small head with a flat or absent nasal bone. The diagnosis is typically made between 14 and 16 weeks of gestation, with the highest accuracy at 14 weeks. This is because the fetal brain and skull are adequately developed at this stage to demonstrate the characteristic features of anencephaly. The ultrasound examination should focus on the cranial vault, the presence or absence of the nasal bone, and the shape of the fetal head.

**Why Each Wrong Option is Incorrect**
**Option A:** At 6 weeks of gestation, the fetal brain and skull are not adequately developed to diagnose anencephaly accurately. The embryo is still in the early stages of development, and the ultrasound resolution is limited.
**Option B:** While some anencephaly cases may be detectable at 8 weeks of gestation, the diagnosis is not as accurate as at 14 weeks due to the incomplete development of the fetal brain and skull.
**Option C:** At 10 weeks of gestation, the fetal brain and skull are beginning to develop, but the diagnosis of anencephaly may not be as clear-cut as at 14 weeks.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that anencephaly is a severe congenital anomaly that requires prompt diagnosis and counseling. The ultrasound examination should focus on the cranial vault, the presence or absence of the nasal bone, and the shape of the fetal head. A diagnosis of anencephaly at 14 weeks of gestation has significant implications for prenatal management and planning.

**✓ Correct Answer: D. 14 weeks of gestation**