All of the following are neural tube defects except-
**Core Concept**
Neural tube defects (NTDs) are congenital anomalies that occur during the early stages of embryonic development, specifically between 23 and 26 days post-fertilization. They result from the incomplete closure of the neural tube, leading to defects in the brain or spinal cord.
**Why the Correct Answer is Right**
The correct answer is likely one of the options that does not involve a defect in the neural tube. Neural tube defects include anencephaly, spina bifida, and encephalocele. These conditions occur due to the failure of the neural tube to close properly, leading to exposed brain tissue or a malformed spinal cord.
**Why Each Wrong Option is Incorrect**
**Option A:** Spina bifida is a neural tube defect characterized by the incomplete closure of the spine, resulting in a protrusion of the meninges and spinal cord through the posterior neural arches.
**Option B:** Anencephaly is a neural tube defect where the upper portion of the neural tube fails to close, resulting in the absence of a significant portion of the brain and skull.
**Option C:** Encephalocele is a neural tube defect where a portion of the brain protrudes through a defect in the skull.
**Clinical Pearl / High-Yield Fact**
It is essential to identify neural tube defects early in pregnancy, as they can have significant implications for fetal development and maternal health. Folic acid supplementation has been shown to reduce the risk of neural tube defects by up to 70%.
**Correct Answer:** D.