**Core Concept**
Teratogens are substances or agents that can cause developmental abnormalities or birth defects in a fetus. Understanding the teratogenic potential of various agents is crucial for preventing and managing pregnancy complications.

**Why the Correct Answer is Right**
The correct answer is a substance that is not known to have teratogenic effects. In the context of pregnancy, many substances can pose risks to fetal development, including certain medications, environmental toxins, and lifestyle factors. However, some substances are considered safe and are not associated with teratogenic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** [Insert substance or agent, e.g. alcohol] is a known teratogen that can cause fetal alcohol spectrum disorders (FASD), a range of developmental and physical problems. Alcohol exposure during pregnancy can lead to growth restriction, facial abnormalities, and cognitive impairment in the child.

**Option B:** [Insert substance or agent, e.g. mercury] is a toxic substance that can cause neurological damage and developmental delays in the fetus. Mercury exposure during pregnancy has been linked to an increased risk of birth defects, including cerebral palsy and hearing loss.

**Option C:** [Insert substance or agent, e.g. folic acid] is actually a teratogenic preventer. Folic acid supplementation is recommended to pregnant women to prevent neural tube defects (NTDs) such as spina bifida and anencephaly.

**Clinical Pearl / High-Yield Fact**
The "TERATOMETER" mnemonic can help medical students remember common teratogens: Tobacco, Ethanol, Radiation, Aspirin, Mercury, Thalidomide, and Other medications.

**Correct Answer:** D.