Which feature best distinguishes phenytoin from valproic acid in terms of their teratogenic effects?

Explanation

## Teratogenic Profiles: Phenytoin vs Valproic Acid ### Phenytoin Teratogenicity **Key Point:** Phenytoin causes **fetal hydantoin syndrome**, a characteristic pattern of congenital abnormalities. Features of fetal hydantoin syndrome: - Facial dysmorphism (broad nasal bridge, hypertelorism, short neck) - Microcephaly and intellectual disability - Cardiac defects (patent ductus arteriosus, atrial septal defect) - Limb hypoplasia and nail/digit hypoplasia - Cleft lip/palate (less common than with valproate) ### Valproic Acid Teratogenicity **Key Point:** Valproic acid causes **neural tube defects (NTDs)** as its primary teratogenic manifestation. Features of valproate embryopathy: - **Neural tube defects** (spina bifida, anencephaly) — 10–20× higher risk than general population - Cardiac defects (atrial septal defect, ventricular septal defect) - Cleft palate - Developmental delay and autism spectrum disorder (emerging evidence) - Urogenital abnormalities ### Comparative Table | Feature | Phenytoin | Valproic Acid | | --- | --- | --- | | **Primary NTD risk** | Low (~0.5%) | High (1–2%) | | **Characteristic syndrome** | Fetal hydantoin syndrome | Valproate embryopathy | | **Facial dysmorphism** | Yes (broad nasal bridge) | Mild | | **Limb hypoplasia** | Yes (nail/digit) | Rare | | **Cardiac defects** | Common | Common | | **Cleft palate** | Occasional | More frequent | | **Neurodevelopmental** | Intellectual disability | Autism, developmental delay | **High-Yield:** The **discriminating feature** is that phenytoin causes fetal hydantoin syndrome (recognizable facial/limb pattern), while valproic acid's hallmark is **neural tube defects**. This distinction is critical for counselling women of childbearing age. **Clinical Pearl:** Valproic acid is now considered **Category X in pregnancy** (or equivalent in newer pregnancy category systems) in many countries due to the high NTD and neurodevelopmental risk. Phenytoin, while teratogenic, is sometimes used when benefits outweigh risks in women already on stable therapy. **Mnemonic:** **PHE-NTD** (Phenytoin = Hydantoin syndrome; Valproate = Neural Tube Defects).