A 3-year-old girl is referred for evaluation of short stature and developmental concerns. On examination, she has a broad chest with widely spaced nipples, low posterior hairline, bilateral ptosis, low-set ears, and mild pulmonic stenosis on echocardiogram. Her chromosomal analysis reveals a 46,XX karyotype. Which of the following additional findings would most strongly support the unifying diagnosis in this child compared with the most phenotypically similar condition in the differential?

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Hypertrophic cardiomyopathy and PTPN11 gene mutation. ***Hypertrophic cardiomyopathy and PTPN11 gene mutation***
  - The presentation of **Noonan syndrome** features a **46,XX karyotype** and **pulmonic stenosis**, which distinguishes it from the phenotypically similar **Turner syndrome**.
  - A mutation in the **PTPN11 gene** and findings of **hypertrophic cardiomyopathy** are highly specific for Noonan syndrome and support the unifying diagnosis.
*Webbed neck and short fourth metacarpal*
  - These features are seen in both **Noonan syndrome** and **Turner syndrome**, making them poor discriminators between the two conditions.
  - While common in dwarfism/skeletal dysplasias, they do not help isolate the **46,XX** Noonan phenotype from the **45,X** Turner phenotype.
*Bicuspid aortic valve and 45,X karyotype*
  - This clinical picture describes **Turner syndrome**, but the patient in the vignette has a normal **46,XX karyotype**.
  - **Bicuspid aortic valve** is a classic left-sided heart defect associated with Turner syndrome, whereas Noonan syndrome typically involves **right-sided defects**.
*Coarctation of the aorta and 45,X karyotype*
  - These findings are pathognomonic for **Turner syndrome** and contradict the patient's documented **46,XX karyotype**.
  - Pulmonary stenosis is rarely seen in Turner syndrome, while **coarctation of the aorta** is a hallmark finding absent in Noonan syndrome.
*Ventricular septal defect and 47,XX+21 karyotype*
  - This karyotype identifies **Down syndrome** (Trisomy 21), which presents with distinct facial features like **epicanthal folds** and **upslanting palpebral fissures**.
  - While **ventricular septal defects** are common in Down syndrome, they do not align with the patient’s observed **ptosis** or **pulmonic stenosis**. [Practice more Pediatrics questions on OnCourse]

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