Left horn of sinus venosus forms:
**Core Concept**
The **sinus venosus** is a critical structure in embryonic heart development, playing a key role in the formation of the atria. It is involved in the incorporation of the **cardinal veins** and **common pulmonary vein** into the heart. The left horn of the sinus venosus contributes to a specific part of the adult heart.
**Why the Correct Answer is Right**
The left horn of the sinus venosus develops into the **coronary sinus**. This is because the coronary sinus is formed from the left common cardinal vein, which is incorporated into the left horn of the sinus venosus during embryonic development. The coronary sinus is a venous structure that returns deoxygenated blood from the myocardium to the right atrium.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the developmental origin of the coronary sinus.
**Option B:** This option is incorrect as it does not correctly identify the structure formed by the left horn of the sinus venosus.
**Option C:** This option is incorrect because the left horn of the sinus venosus does not develop into this structure.
**Clinical Pearl / High-Yield Fact**
Remembering that the left horn of the sinus venosus forms the coronary sinus is crucial for understanding cardiac embryology and its clinical implications, such as the origin of coronary sinus-related anomalies.
**Correct Answer:** D. Coronary sinus.