This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Anatomy
[{'content': 'What is seen earliest in USG? \nSelect and explain. \nA. Yolk sac, B. Fetal hea, C. Chorion, D. Placenta', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Yolk sac is the first anatomic structure seen within gestational sac. By TVS, it can be seen as early as 5.5week By TAS, it is seen by 7 weeks. Ref:Datta Obs 9e pg 601.', 'role': 'assistant'}]