**Core Concept:** The interventricular septum is a part of the heart's structure. It is the area within the heart that separates the left and right ventricles. In humans, the interventricular septum is mostly composed of myocardium and is essential for proper heart function.

**Why the Correct Answer is Right:** The interventricular septum is formed by the fusion of two layers of mesoderm during embryonic development: the cardinal venous cushions and the outflow tract myocardium. The cardinal venous cushions are the remnants of the embryonic heart tube and contribute to the formation of the arterial ring, while the outflow tract myocardium forms the bulk of the interventricular septum.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the interventricular septum is not primarily formed by the outflow tract myocardium. Instead, it is primarily formed by the fusion of cardinal venous cushions and outflow tract myocardium during embryonic development.

B. This option is incorrect because the interventricular septum is not primarily formed by the right atrioventricular cushions. The correct formation process involves the fusion of cardinal venous cushions and outflow tract myocardium.

C. This option is incorrect because the interventricular septum is not primarily formed by the left atrioventricular cushions. As mentioned earlier, the correct formation process involves the fusion of cardinal venous cushions and outflow tract myocardium.

D. This option is incorrect because the interventricular septum is not primarily formed by the coronary sinus. The interventricular septum is primarily formed by the fusion of cardinal venous cushions and outflow tract myocardium during embryonic development.

**Clinical Pearl:** Understanding the development of the interventricular septum is essential for understanding congenital heart defects, such as ventricular septal defects (VSDs), which occur when the septum is incompletely formed or malformed during development.

**Correct Answer:** D. The coronary sinus is a vessel in the heart that collects blood from the right atrium and right ventricle. It is not involved in the formation of the interventricular septum. The interventricular septum is primarily formed by the fusion of cardinal venous cushions and outflow tract myocardium during embryonic development.