A 3-month-old male infant with Down syndrome (trisomy 21) is being compared with a 3-month-old female with Edwards syndrome (trisomy 18) during a pediatric genetics clinic. Which feature is the BEST discriminator between these two aneuploidies?

## Distinguishing Down Syndrome from Edwards Syndrome ### Key Discriminating Features **Key Point:** While both trisomy 21 (Down) and trisomy 18 (Edwards) present with multiple congenital anomalies and developmental delay, their **facial and limb dysmorphology** are distinctly different and allow clinical differentiation. ### Comparison Table | Feature | Down Syndrome (Trisomy 21) | Edwards Syndrome (Trisomy 18) | |---------|---------------------------|-------------------------------| | **Facial features** | Flat facies, broad nose, upslanting palpebral fissures, epicanthal folds | Prominent occiput, micrognathia, low-set ears, rocker-bottom feet | | **Hand deformity** | Single palmar crease (simian crease) | Clenched fists with overlapping fingers (index over 3rd, 5th over 4th) | | **Cardiac defects** | AV canal defect, VSD, ASD (60%) | VSD, PDA, polyvalvular disease (90%) | | **Prognosis** | Survival to adulthood common (>50 years) | Usually fatal by age 1 year (median 5–15 days) | | **Intellectual disability** | Mild to moderate (IQ 30–70) | Severe | ### Clinical Pearl **Clinical Pearl:** The **rocker-bottom feet** (convex sole) and **clenched fists with characteristic finger overlap** are pathognomonic for Edwards syndrome and are rarely seen in Down syndrome. Conversely, the **single palmar crease** and **upslanting palpebral fissures** are hallmark features of Down syndrome. ### High-Yield Mnemonic **Mnemonic — "EDWARDS ROCKS":** - **E**dwards: rocker-bottom feet - **D**own: flat facies, single palmar crease - **W**ards: overlapping fingers (clenched fists) - **A**rthrography: joint contractures in Edwards - **R**ocker: rocker-bottom feet (Edwards) - **D**own: upslanting palpebral fissures - **S**imian crease: Down syndrome ### Why the Correct Option Works Option B (flat facies with upslanting palpebral fissures and a single palmar crease) is the **best discriminator** because these three features together are **diagnostic of Down syndrome** and are **absent in Edwards syndrome**. This combination allows immediate clinical differentiation. [cite:Park 26e Ch 3]