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Noonan syndrome – male Turner phenotype — RACP Paediatrics MCQ

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ModerateGeneticsNoonan syndrome – male Turner phenotypeRACP Paediatrics

A neonate has pulmonary valve stenosis, short stature, webbed neck, low posterior hairline, widely spaced nipples, cryptorchidism, and pectus excavatum. The phenotype resembles Turner syndrome but the karyotype is 46,XY. What is the most likely diagnosis?

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Correct answer: E — Noonan syndrome

Noonan syndrome shares phenotypic features with Turner syndrome (short stature, webbed neck, widely spaced nipples, cardiac defects) but occurs in both sexes with a normal karyotype. Pulmonary stenosis is the most common cardiac defect (unlike Turner where it's coarctation/bicuspid AV). It is caused by RASopathy gene mutations (PTPN11 most common, autosomal dominant).

Reference: RACP Paediatric Curriculum – Genetics; RCH Melbourne – 2023