**Core Concept**
The question is testing the knowledge of teratogenic agents in pregnancy, specifically their potential to cause congenital abnormalities in the developing fetus. Teratogens are substances or factors that can interfere with fetal development, leading to birth defects.

**Why the Correct Answer is Right**
The correct answer is **isotretinoin** (Accutane), a retinoid derivative used for the treatment of severe acne. It is a potent teratogen that can cause severe congenital anomalies, including craniofacial abnormalities, heart defects, and central nervous system malformations. Isotretinoin works by inhibiting the binding of retinoic acid to its receptors, which are essential for normal embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** **Warfarin** is a teratogen that can cause fetal warfarin syndrome, characterized by nasal hypoplasia and stippling of the bones. However, it is not as severe a teratogen as isotretinoin.
**Option B:** **Phenytoin** is an antiepileptic drug that can cause fetal hydantoin syndrome, which includes features such as cleft lip and palate, heart defects, and developmental delay. While it is a teratogen, it is not as potent as isotretinoin.
**Option C:** **Thalidomide** is a classic teratogen that can cause severe limb abnormalities and other congenital defects. However, it is not as commonly used as isotretinoin, and its use is heavily restricted due to its teratogenic potential.
**Option D:** **Aspirin** is not typically considered a teratogen, and its use in pregnancy is generally not associated with an increased risk of congenital anomalies.

**Clinical Pearl / High-Yield Fact**
Isotretinoin is so potent that even a single dose during pregnancy can cause severe congenital anomalies. Therefore, it is essential to use strict precautions, including pregnancy testing and contraception, when prescribing isotretinoin to women of childbearing age.

**Correct Answer:** C. Thalidomide