**Core Concept**
Noonan syndrome is a genetic disorder characterized by a distinct set of physical and developmental features, including congenital heart defects, short stature, and facial abnormalities. It is often referred to as a "male version" of Turner syndrome. The syndrome is caused by mutations in the PTPN11 gene, which encodes a protein involved in signal transduction pathways.

**Why the Correct Answer is Right**
Coarctation of the aorta is not a typical feature of Noonan syndrome. Instead, patients with Noonan syndrome often present with pulmonary valve stenosis, which can be dysplastic or supravalvar. Hypertrophic cardiomyopathy is also a common cardiac manifestation of the syndrome. The characteristic cardiac features of Noonan syndrome are a result of the abnormal signaling pathways mediated by the mutated PTPN11 protein, which affects the development of the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypertrophic cardiomyopathy is indeed a common feature of Noonan syndrome, making it an incorrect answer.
**Option B:** Dysplastic pulmonary stenosis is a characteristic cardiac feature of Noonan syndrome, often presenting with a supravalvar or valvar component. This option is incorrect because it is a typical manifestation of the syndrome.
**Option C:** Pectus excavatum is a common skeletal feature of Noonan syndrome, often co-occurring with other skeletal abnormalities such as short stature and joint hypermobility. This option is incorrect because it is a characteristic feature of the syndrome.

**Clinical Pearl / High-Yield Fact**
Noonan syndrome is often associated with a characteristic facial appearance, which includes a webbed neck, low-set ears, and a prominent forehead. This facial phenotype can be an important clue in the diagnosis of the syndrome.

**✓ Correct Answer: D. Coarctation of aorta**