**Core Concept**
Anencephaly is a congenital anomaly characterized by the absence of a major portion of the brain, skull, and scalp. It occurs due to the failure of the neural tube to close during fetal development.

**Why the Correct Answer is Right**
Anencephaly is the earliest detectable fetal anomaly by ultrasound (USG) due to its distinct appearance. The absence of the cranial vault and the presence of a small, misshapen head with a poorly formed scalp make it easily identifiable. The neural tube defect occurs during the 23-26th day of gestation, and its detection is possible as early as 10-14 weeks of gestation. The absence of the brain tissue and the presence of a thin, membrane-covered mass in the cranial region are characteristic features of anencephaly.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocephalus, or an accumulation of cerebrospinal fluid in the brain, can be detected by USG but is not the earliest anomaly. It can be identified as early as 15-16 weeks of gestation.

**Option B:** Achondroplasia, a form of short-limbed dwarfism, is a genetic disorder that affects bone growth. While it can be detected by USG, it is not the earliest anomaly and is typically identified later in pregnancy, around 16-20 weeks.

**Option D:** Spina bifida, a congenital defect of the spine, can be detected by USG but is not the earliest anomaly. It is typically identified later in pregnancy, around 16-20 weeks.

**Clinical Pearl / High-Yield Fact**
Anencephaly is a severe and usually fatal congenital anomaly, with a high mortality rate. It is essential for obstetricians to identify it early in pregnancy to provide appropriate counseling and management options for the parents.

**✓ Correct Answer: C. Anencephaly**