Clinical Scenario:
A 6-year-old girl is evaluated for short stature. She has a broad chest with widely spaced nipples, a low posterior hairline, and a short, broad neck. Echocardiography demonstrates coarctation of the aorta, while renal ultrasonography shows a horseshoe kidney. Her karyotype is pending. Noonan syndrome is considered because of the overlapping phenotypic features.
Which of the following findings developing later in adolescence would most strongly support Turner syndrome rather than Noonan syndrome?
A. Pulmonary valve stenosis
B. Hypertrophic cardiomyopathy
C. Hypergonadotropic hypogonadism with streak gonads
D. Mild ptosis with refractive error
E. Normal spontaneous pubertal development
Correct answer & Explanation:
Answer: C. Hypergonadotropic hypogonadism with streak gonads
Discussion
This girl has several characteristic features of Turner syndrome, including short stature, broad chest with widely spaced nipples, short/broad neck, coarctation of the aorta, and a renal anomaly.
Turner syndrome results from complete or partial loss of one X chromosome and is associated with gonadal dysgenesis. The ovaries are typically replaced by streak gonads, resulting in ovarian insufficiency and hypergonadotropic hypogonadism. Consequently, affected girls commonly develop absent or incomplete spontaneous puberty and primary amenorrhea.
Noonan syndrome can phenotypically resemble Turner syndrome and is caused by pathogenic variants affecting the RAS/MAPK signaling pathway. However, its characteristic cardiac abnormalities are pulmonary valve stenosis and hypertrophic cardiomyopathy. Females with Noonan syndrome generally have preserved ovarian function and can undergo spontaneous puberty.
The combination of coarctation of the aorta + short stature + characteristic phenotype + gonadal dysgenesis strongly favors Turner syndrome.
Why the other options are less appropriate:
- A. Pulmonary valve stenosis: strongly associated with Noonan syndrome.
- B. Hypertrophic cardiomyopathy: a characteristic cardiac manifestation of Noonan syndrome.
- D. Ptosis: commonly described in Noonan syndrome.
- E. Normal spontaneous puberty: argues against Turner syndrome.
CPSP exam pearl:
When Turner syndrome and Noonan syndrome overlap phenotypically, remember the endocrine distinction: Turner → streak gonads → hypergonadotropic hypogonadism, whereas females with Noonan syndrome usually have normal spontaneous pubertal development.
