**Core Concept**
Anencephaly is a congenital anomaly characterized by the absence of a major portion of the brain and skull. Early diagnosis is crucial for counseling and planning the management of the pregnancy.

**Why the Correct Answer is Right**
USG (ultrasound) is the best diagnostic tool for anencephaly due to its high sensitivity and specificity. It allows for direct visualization of the fetal skull and brain, enabling the detection of characteristic features such as the absence or significant reduction of the cranial vault, open eyes, and an absent or severely hypoplastic cerebellum. Transabdominal or transvaginal ultrasound can be used, depending on the gestational age and fetal position.

**Why Each Wrong Option is Incorrect**
**Option A:** Maternal Alpha-fetoprotein (AFP) levels may be elevated in anencephaly due to the increased amount of fetal neural tissue, but this is not a specific or sensitive test for diagnosis. Elevated AFP levels can also be associated with other fetal anomalies and conditions.
**Option B:** Amniotic fluid beta-hCG levels are not directly related to the diagnosis of anencephaly. While beta-hCG levels may be used to assess the risk of chromosomal abnormalities, they are not a diagnostic tool for structural anomalies like anencephaly.
**Option D:** X-ray is not a suitable diagnostic tool for anencephaly, especially in pregnancy, due to the risks associated with radiation exposure to the fetus. Additionally, X-ray may not provide sufficient detail for the diagnosis of anencephaly.

**Clinical Pearl / High-Yield Fact**
USG should be performed between 16 and 20 weeks of gestation for optimal visualization of fetal anatomy, including the brain and skull. Early detection of anencephaly allows for counseling and planning for potential termination of pregnancy.

✓ Correct Answer: C. USG