Comparison Table: Patau Syndrome (Trisomy 13) — High-Yield USMLE Genetics
Patau syndrome (Trisomy 13) is a classic autosomal trisomy tested on USMLE for its distinct congenital anomalies, etiology (meiotic nondisjunction vs Robertsonian translocation), and poor survival.
The 1-Liner (USMLE-ready)
Patau syndrome (Trisomy 13) = severe midline defects (e.g., holoprosencephaly, cleft lip/palate) + polydactyly + rocker-bottom feet + congenital heart disease, most often due to meiotic nondisjunction.
Visual / Mnemonic Device
“13 = Unlucky → ‘PA-TAU’ = ‘PA’late + ‘TA’il (polydactyly) + ‘U’nderdeveloped midline brain”
Think midline failure:
- PAlate problem → cleft lip/palate
- TAil extras → polydactyly
- Underdeveloped brain → holoprosencephaly / severe intellectual disability
Quick Comparison Table (Patau vs Edwards vs Down)
| Feature | Patau (Trisomy 13) | Edwards (Trisomy 18) | Down (Trisomy 21) |
|---|---|---|---|
| Key idea | Midline defects + polydactyly | Clenched hands + rocker-bottom feet | Characteristic facies + hypotonia |
| Classic findings | Holoprosencephaly, cleft lip/palate, polydactyly, microphthalmia, cutis aplasia (scalp defects) | Prominent occiput, micrognathia, low-set ears, clenched fists with overlapping fingers, rocker-bottom feet | Upward slanting palpebral fissures, epicanthal folds, single palmar crease, sandal gap, Brushfield spots |
| Heart defects | VSD/ASD/PDA (common) | VSD (common) | AV septal defect (esp. endocardial cushion defects) |
| Other high-yield associations | Omphalocele, severe ID, seizures | Severe growth restriction, renal anomalies | Duodenal atresia, Hirschsprung, ↑ risk ALL/AML, early Alzheimer, hypothyroidism |
| Most common cause | Maternal meiotic nondisjunction | Maternal meiotic nondisjunction | Maternal meiotic nondisjunction (majority), Robertsonian translocation (some) |
| Risk factor | Advanced maternal age | Advanced maternal age | Advanced maternal age |
| Prognosis | Very poor, often death in days–weeks (many within 1st year) | Very poor, many die within 1st year | Variable; many survive to adulthood with support |
| Karyotype | 47,XX/XY,+13 | 47,XX/XY,+18 | 47,XX/XY,+21 |
Patau Syndrome (Trisomy 13): High-Yield Fact Sheet
Core clinical features (know these cold)
- CNS / midline: holoprosencephaly, severe intellectual disability, seizures
- Face/eyes: cleft lip/palate, microphthalmia
- Limbs: polydactyly
- Skin: cutis aplasia (focal scalp defects)
- Abdominal wall: omphalocele
- Cardiac: VSD/ASD/PDA (congenital heart disease is common)
Genetics & mechanism (Step 1–relevant)
- Usually due to maternal meiotic nondisjunction
- Less commonly due to Robertsonian translocation
- Testable pearl: If a trisomy is due to a balanced translocation in a parent, recurrence risk is higher than sporadic nondisjunction.
How It Shows Up on Exams (Typical Vignettes)
Look for a newborn with:
- Cleft lip/palate + polydactyly
- Severe neurologic impairment / holoprosencephaly
- Congenital heart disease
- Omphalocele or scalp defects …and a stem mentioning advanced maternal age.
Rapid “Differentiate in 5 Seconds”
- Trisomy 13 (Patau) → Polydactyly + Aprosencephaly/holoprosencephaly + cleft PAlate
- Trisomy 18 (Edwards) → Excessive flexion (clenched hands) + rocker-bottom feet
- Trisomy 21 (Down) → AV septal defect + duodenal atresia + characteristic facies