Anencephaly is best diagnosed at 12 weeks by :
**Core Concept**
Anencephaly is a congenital anomaly characterized by the absence of a significant portion of the brain, skull, and scalp. It occurs due to the failure of the neural tube to close properly during embryonic development.
**Why the Correct Answer is Right**
Anencephaly can be diagnosed as early as 10-14 weeks of gestation by ultrasound examination. The characteristic finding is the absence of the cranial vault, and the presence of a small, underdeveloped brain. **Transvaginal ultrasound** is particularly useful for detecting anencephaly, as it provides a closer view of the fetus. This technique allows for the accurate visualization of the fetal anatomy and the identification of associated anomalies.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the diagnostic modality or the gestational age for diagnosing anencephaly.
**Option B:** This option is incorrect as it does not accurately describe the diagnostic approach for anencephaly.
**Option C:** This option is incorrect as it does not specify the diagnostic modality or the gestational age for diagnosing anencephaly.
**Clinical Pearl / High-Yield Fact**
Anencephaly is often associated with other neural tube defects, such as spina bifida. It is essential to perform a thorough ultrasound examination to rule out these associated anomalies.
**Correct Answer: D. Transvaginal ultrasound.**