A 20-week anomaly scan is performed on a 28-year-old primigravida. Multiple fetal anomalies are noted including ventriculomegaly, cardiac defect, and cleft lip. Which chromosomal abnormality is the most common cause of this constellation of findings?

Explanation

## Chromosomal Abnormalities and Fetal Anomalies ### Most Common Cause of Ventriculomegaly + Cardiac Defect + Cleft Lip **Key Point:** Trisomy 13 (Patau syndrome) is the chromosomal abnormality most classically associated with the triad of **ventriculomegaly/holoprosencephaly, cardiac defects, and cleft lip/palate** on prenatal ultrasound. ### Comparison of Trisomies on Prenatal USG | Feature | Trisomy 21 | Trisomy 18 | Trisomy 13 | |---------|-----------|-----------|----------| | **CNS anomaly** | Mild ventriculomegaly | Choroid plexus cysts, mild ventriculomegaly | Holoprosencephaly / ventriculomegaly (severe, hallmark) | | **Cardiac defects** | AV canal, VSD | VSD, PDA, ASD | VSD, PDA (>80%) | | **Cleft lip/palate** | Rare | Occasional | **Very common (60–80%)** | | **Clenched fists / overlapping fingers** | Absent | **Present (hallmark)** | Absent | | **Rocker-bottom feet** | Absent | Present | Present | | **Holoprosencephaly** | Absent | Rare | **Common (hallmark)** | | **Polydactyly** | Absent | Absent | **Present (postaxial, hallmark)** | | **Cyclopia / proboscis** | Absent | Absent | Present | | **Viability at term** | 90% | 5–10% | <5% | **High-Yield:** The combination of **ventriculomegaly (or holoprosencephaly) + cardiac defect + cleft lip/palate** is the classic triad of **Trisomy 13 (Patau syndrome)**. Trisomy 18 is distinguished by overlapping/clenched fingers and rocker-bottom feet, but cleft lip is far more characteristic of Trisomy 13. ### Why Not Trisomy 18? Trisomy 18 (Edwards syndrome) does feature cardiac defects and can have cleft lip, but its **hallmark** ultrasound findings are **clenched fists with overlapping fingers, rocker-bottom feet, and choroid plexus cysts** — not the triad described in this stem. Cleft lip/palate occurs in ~15% of Trisomy 18 cases vs. ~60–80% in Trisomy 13. ### Clinical Pearl **High-Yield:** The mnemonic for Trisomy 13 features — **"Patau = Palate (cleft), Polydactyly, Prosencephaly (holoprosencephaly)"** — captures the three most distinguishing prenatal ultrasound findings. When a question pairs ventriculomegaly/CNS anomaly + cardiac defect + cleft lip, Trisomy 13 is the answer. ### Mnemonic for Trisomy 13 Features **"PATAU"**: - **P**rosencephaly (holoprosencephaly) / ventriculomegaly - **A**nomalies of the heart (VSD, PDA — >80%) - **T**riangle face with cleft lip/palate - **A**dditional digits (postaxial polydactyly) - **U**mbilical hernia / omphalocele [cite: Callen's Ultrasonography in Obstetrics and Gynecology 6e Ch 9; Sadler's Langman's Medical Embryology 14e]