**Core Concept**
The right atrium receives deoxygenated blood from the body through the superior and inferior vena cavae, and it also receives oxygen-rich blood from the coronary sinus. The coronary sinus is a venous structure that collects deoxygenated blood from the myocardium.

**Why the Correct Answer is Right**
The coronary sinus is a venous structure that collects deoxygenated blood from the myocardium and opens directly into the right atrium. This allows the right atrium to receive deoxygenated blood from the myocardium, which is then pumped to the right ventricle and eventually to the lungs for oxygenation. The coronary sinus is formed by the union of small cardiac veins that drain the myocardium.

**Why Each Wrong Option is Incorrect**
* **Option A:** The superior vena cava is a major vein that returns deoxygenated blood from the upper body to the right atrium, but it does not open into the right atrium directly; rather, it opens into the coronary sinus, which then opens into the right atrium.
* **Option B:** The inferior vena cava is a major vein that returns deoxygenated blood from the lower body to the right atrium, but it does not open into the right atrium directly; rather, it opens into the coronary sinus, which then opens into the right atrium.
* **Option D:** The pulmonary veins return oxygen-rich blood from the lungs to the left atrium, not the right atrium.

**Clinical Pearl / High-Yield Fact**
The coronary sinus is a critical structure in the cardiac venous system, and its obstruction can lead to cardiac ischemia and infarction.

**Correct Answer: C. Coronary sinus.**