**Core Concept**
Anencephaly is a congenital anomaly characterized by the absence of a significant portion of the brain, skull, and scalp, resulting from a failure of neural tube closure during embryonic development.

**Why the Correct Answer is Right**
Accurate diagnosis of anencephaly on ultrasound is critical for prenatal counseling and management. The characteristic ultrasound features of anencephaly include the absence of the cranial vault, a small or absent bony calvaria, and a prominent cerebellum and brainstem. The ultrasound can detect these abnormalities as early as 10-14 weeks of gestation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because anencephaly is not typically diagnosed through physical examination, as the absence of cranial vault and scalp tissue prevents palpation of the fetal skull.
**Option B:** This option is incorrect because MRI is not the primary imaging modality for diagnosing anencephaly, although it may be used in some cases for further evaluation or to assess fetal anatomy.
**Option C:** This option is incorrect because amniocentesis is a diagnostic procedure used to detect chromosomal abnormalities and other genetic disorders, but it is not used for the diagnosis of anencephaly.

**Clinical Pearl / High-Yield Fact**
The "lemon sign" and "banana sign" are two important ultrasound markers of anencephaly, which can be detected in the second trimester of pregnancy.

**Correct Answer:** C. Amniocentesis