**Core Concept**
Anencephaly is a congenital anomaly characterized by the absence of a significant portion of the brain, skull, and scalp. It is a type of neural tube defect (NTD) that occurs during embryonic development.

**Why the Correct Answer is Right**
The diagnosis of anencephaly by ultrasound (USG) is based on the visualization of the absence of the cranial vault and the presence of the brain tissue at the base of the skull. This is often accompanied by the presence of a small, open neural plate, which is a characteristic feature of anencephaly. The ultrasound examination typically reveals a lack of the cranial bones and a small, open cranial cavity, allowing for the visualization of the underlying brain tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific gestational age for the diagnosis of anencephaly. While anencephaly can be diagnosed as early as 10-14 weeks of gestation, the exact timing may vary depending on the skill of the sonographer and the quality of the ultrasound equipment.

**Option B:** This option is incorrect because it suggests that anencephaly can be diagnosed by USG at 24 weeks of gestation. While anencephaly can be diagnosed at this gestational age, it is not the earliest possible time for diagnosis.

**Option C:** This option is incorrect because it suggests that anencephaly can be diagnosed by USG at 32 weeks of gestation. This is not the earliest possible time for diagnosis, and the option is incorrect.

**Clinical Pearl / High-Yield Fact**
The diagnosis of anencephaly by USG is often confirmed by the presence of a small, open neural plate and the absence of the cranial vault. It is essential to perform a thorough ultrasound examination and to correlate the findings with other diagnostic tests, such as maternal serum alpha-fetoprotein (MSAFP) levels, to confirm the diagnosis.

**Correct Answer: A. 10-14 weeks.**