## **Core Concept**
The right atrium receives deoxygenated blood from the body through several venous structures. The main chambers and vessels that drain into the right atrium include the **superior vena cava**, **inferior vena cava**, **coronary sinus**, and **thebesian veins**. Understanding the normal anatomy and connections to the right atrium is crucial for identifying any anomalies.

## **Why the Correct Answer is Right**
The **coronary sinus** primarily opens into the right atrium. The **superior vena cava** and **inferior vena cava** are major systemic veins that return deoxygenated blood from the upper and lower parts of the body, respectively, directly into the right atrium. The **thebesian veins** are small and drain the myocardium into the right atrium and, to a lesser extent, into the right ventricle.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **superior vena cava** does open into the right atrium, making it an incorrect choice as an exception.
- **Option B:** The **inferior vena cava** also opens into the right atrium, which means it is not the correct exception.
- **Option D:** The **coronary sinus** primarily opens into the right atrium, so it's not the exception either.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is the **sinus venarum**, which is a part of the right atrium derived from the embryonic sinus venosus and receives blood from the **superior vena cava**, **inferior vena cava**, and **coronary sinus**.

## **Correct Answer:** .