**Core Concept**
Anencephaly is a severe 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 during embryonic development. Ultrasound is a non-invasive imaging modality used to detect anencephaly during fetal development.

**Why the Correct Answer is Right**
Anencephaly can be diagnosed by ultrasound as early as 10-14 weeks of gestation. The characteristic absence of the cerebral hemispheres, cranial vault, and scalp, along with the presence of a small head circumference, can be visualized using high-resolution ultrasound equipment. The presence of a "lemon sign" (an upward displacement of the frontal bone) and a "banana sign" (a curved appearance of the cerebellum) are also indicative of anencephaly.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because anencephaly cannot be diagnosed by palpation alone, as it requires imaging modalities like ultrasound or MRI for accurate diagnosis.
* **Option B:** This option is incorrect because while anencephaly can be associated with other congenital anomalies, it is not exclusively diagnosed by the presence of polydactyly (extra fingers or toes).
* **Option C:** This option is incorrect because while anencephaly can be associated with hydrocephalus, the absence of the cerebral hemispheres and cranial vault is a more specific indicator of the condition.

**Clinical Pearl / High-Yield Fact**
Anencephaly is a marker for an increased risk of other congenital anomalies, including cardiac defects, cleft palate, and clubfoot. A careful examination of fetal anatomy and a detailed ultrasound scan can help identify these associated anomalies.

**Correct Answer: B. 18-20 weeks of gestation**