What is a key characteristic of Twin Reversed Arterial Perfusion Syndrome (TRAP)?

Twin Reversed Arterial Perfusion Syndrome (TRAP) is characterized by a particular hemodynamic situation in which blood flow is altered between twins. The correct answer focuses on the retrograde arterial flow to the acardiac twin, which is integral to the pathophysiology of TRAP. In this syndrome, one twin, referred to as the "pump twin," provides blood flow to the other twin, known as the "acardiac twin." This acardiac twin lacks a heart and fails to develop normally, relying on the pumping action of the other twin's heart for its blood supply, but this flow is retrograde. This means that instead of a normal blood flow pattern, the arterial blood from the pump twin flows backward into the acardiac twin, which is a defining aspect of TRAP.

This retrograde flow occurs due to the shared placental circulation that defines monochorionic twin pregnancies. Although it is true that TRAP occurs in monochorionic twins, the key characteristic that distinguishes TRAP is this abnormal retrograde perfusion pattern, making the answer that focuses on retrograde arterial flow to the acardiac twin the most accurate reflection of the syndrome's nature.