This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
[{'content': 'Which of the following is not echogenic on ultrasonography- \nSelect and explain. \nA. Bile, B. Gas, C. Bone, D. Gall Stones', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Echoes are not produced if there is no difference in a tissue or between tissues. Homogenous fluids like blood, bile, urine, contents of simple cysts, ascites and pleural effusion are seen as echo-free structures. Ref : Physical principles of ultrasound (Dr Daniel J Bell and Ass. Pr. Mirjan M. Nadrljanski et al.)', 'role': 'assistant'}]