**Core Concept**
The ability of a drug to cross the placenta depends on its molecular weight, lipid solubility, and the presence of charged groups. Lipid-soluble, non-ionized molecules with a molecular weight less than 500 Da can easily cross the placenta, potentially affecting fetal development.

**Why the Correct Answer is Right**
The placenta is a highly selective barrier that allows certain substances to pass through while restricting others. Lipid-soluble, non-ionized molecules like **steroids**, **alcohols**, and **volatile anesthetics** can diffuse across the placenta. However, **proteins** and **molecules with a high molecular weight** are too large to cross the placental barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because **steroids** can easily cross the placenta due to their lipid solubility.
**Option B:** Incorrect because **alcohols** can also cross the placenta and may cause fetal alcohol syndrome.
**Option C:** Incorrect because **volatile anesthetics** can diffuse across the placenta and affect fetal development.
**Option D:** Incorrect because **proteins** are too large to cross the placental barrier.

**Clinical Pearl / High-Yield Fact**
When prescribing medications during pregnancy, it's essential to consider the potential effects on the fetus. Always check the medication's safety profile and potential for placental transfer.

**Correct Answer:** D. Proteins.