A 2-day-old male neonate is admitted with severe congenital anomalies including bilateral cleft lip and palate, postaxial polydactyly, microphthalmia, and a ventricular septal defect. Karyotype analysis shows the notation marked **D** in the diagram. Which of the following clinical features is MOST characteristic of this chromosomal abnormality and would be expected on physical examination?

Explanation

## Why Aplasia cutis congenita on the scalp with holoprosencephaly on neuroimaging is right The notation **D** (47,XY,+13) represents Patau syndrome (Trisomy 13), the third most common autosomal trisomy with an incidence of ~1:10,000–16,000 live births. The clinical anchor is the characteristic "midline fusion defects" triad: holoprosencephaly (failure of the forebrain to divide, ranging from alobar with single ventricle and fused thalami to lobar forms), cleft lip/palate, and polydactyly. A pathognomonic finding in Trisomy 13 is **aplasia cutis congenita** — focal absence of skin on the scalp — which is a highly specific marker for this condition. The combination of holoprosencephaly on neuroimaging with aplasia cutis congenita is the most characteristic presentation (Nelson 21e; Harrison 21e Ch 471). ## Why each distractor is wrong - **Rocker-bottom feet with overlapping fingers and cardiac defects**: This constellation is characteristic of Trisomy 18 (Edwards syndrome), not Trisomy 13. Rocker-bottom feet and overlapping fingers are hallmark features of T18, whereas Trisomy 13 presents with postaxial polydactyly (extra digit on the ulnar side), not overlapping fingers. - **Micrognatia with omphalocele and renal agenesis**: While omphalocele can occur in Trisomy 13 (as part of GI anomalies), the combination of micrognatia with renal agenesis is more typical of other syndromes (e.g., Pierre Robin sequence or other genetic conditions). Trisomy 13 typically presents with polycystic kidneys and hydronephrosis, not agenesis, and micrognatia is not a characteristic feature. - **Hypoplastic left heart with intrauterine growth restriction and low-set ears**: Hypoplastic left heart is more commonly associated with Trisomy 18 and other conditions. While intrauterine growth restriction can occur in Trisomy 13, the specific combination with hypoplastic left heart and low-set ears is not the classic presentation; Trisomy 13 more typically presents with VSD, ASD, or PDA (~80% have congenital heart disease). **High-Yield:** Trisomy 13 = **aplasia cutis congenita + holoprosencephaly + postaxial polydactyly + cleft lip/palate** — the "midline fusion defect" triad with characteristic scalp skin defect. [cite: Nelson 21e; Harrison 21e Ch 471]