This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Embryology 2, Subject name: Anatomy
[{'content': 'Fetal sex can be diagnosed by noting the presence or absence of the Barr body in cells obtained from the amniotic fluid. What is the etiology of the Barr body? \nSelect and explain. \nA. Inactivation of both X chromosomes, B. Inactivation of homologous chromosomes, C. Inactivation of one Y chromosome, D. Inactivation of one X chromosome', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The Barr body is formed from inactivation of one X chromosome in a female. The female has evolved a mechanism for permanent inactivation of one of the X chromosomes presumably because a double dose of X chromosome products would be lethal.', 'role': 'assistant'}]