**Core Concept**
Neural tube defects (NTDs) such as spina bifida and anencephaly are congenital anomalies that occur during early embryonic development. Folic acid supplementation is a well-established prevention strategy to reduce the risk of recurrence in subsequent pregnancies.

**Why the Correct Answer is Right**
The mechanism of folic acid in preventing NTDs involves the methylation of homocysteine to form methionine, which is then converted to S-adenosylmethionine (SAMe) for DNA methylation and neural tube closure. Folic acid deficiency leads to elevated homocysteine levels, disrupting this metabolic pathway. Supplementation with 4-5 mg of folic acid per day is recommended for women with a history of NTDs or those who are planning pregnancy, starting at least 1 month before conception.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the recommended dose of folic acid for NTD prevention is higher than 0.4 mg. Although this dose is sufficient for preventing megaloblastic anemia, it is not sufficient for preventing NTDs.
**Option B:** This option is incorrect because the recommended dose of folic acid for NTD prevention is not 1 mg. While folic acid is essential for preventing NTDs, 1 mg is not the optimal dose.
**Option C:** This option is incorrect because the recommended dose of folic acid for NTD prevention is not 10 mg. Although high doses of folic acid may be used in specific cases, 10 mg is not the recommended dose for preventing NTDs.

**Clinical Pearl / High-Yield Fact**
It is essential to note that folic acid supplementation should be started at least 1 month before conception and continued throughout the first trimester to prevent NTDs.

**Correct Answer:** C. 4-5 mg