This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
[{'content': 'Most sensitive sonological indicator for aneuploidy is \nSelect and explain. \nA. Gestational sac volume, B. Crown-rump length, C. Nuchal transluscency, D. Serum P HCG level', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. is \'c\' i.e. Nuchal translucency Rumack writes - "The most effective sonographic marker of trisomy 21 and other chromosomal defects is increased nuchal translucency (NT) thickness at 11 to 14 weeks gestation."', 'role': 'assistant'}]