This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
Left horn of sinus venosus forms: 
Select and explain. 
A. Coronary sinus, B. Smooth part of right atrium, C. Superior vena cava, D. Inferior vena cava
A: Coronary sinus
Answer: A: Coronary sinus 
Explanation: Ans. A Coronary sinusRef: Netter's Atlas of Human Embryology: 2012 ed. pg. 96Heart tube embryonic derivativesembryonic structureGives rise to* Proximal 1/3rd of bulbus cordisPrimitive trabeculated left ventricle* Middle 1/3rd of bulbus cordisRight and left ventricular outflow tract* Distal 1/3rd of bulbus cordis (truncus arteriosus)Ascending aorta and pulmonary trunk* Left horn of sinus venosusCoronary sinus* Right horn of sinus venosusSmooth part of right atrium* Right common cardinal nerve and right anterior cardinal nerveSVC (superior vena cava)