What defines hypoplastic left heart syndrome?

Hypoplastic left heart syndrome (HLHS) is characterized by the underdevelopment of the left heart structures, which include the left ventricle, aortic valve, aorta, and mitral valve. In HLHS, these components are not fully formed, leading to insufficient blood flow to the body due to the heart's inability to pump effectively through the left side.

The significance of this underdevelopment is that it impacts the systemic circulation, which can lead to critical health issues shortly after birth. Infants with HLHS often require immediate medical intervention and may undergo a series of surgical procedures to manage the condition effectively.

In the context of the other options, overdevelopment of left heart structures would contradict the nature of HLHS. A complete absence of the aorta is not a defining feature of HLHS, as there is typically some form of the aorta present, albeit often hypoplastic. Lastly, normal development of left heart structures describes a healthy heart, which is not applicable to HLHS, where the defining characteristic is impaired development.