This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'Ca-wheel appearance on USG is seen in: \nSelect and explain. \nA. Hydatid cyst, B. Intussusception, C. Hydatidiform mole, D. All', 'role': 'user'}, {'content': "Answer: A \nExplanation: Hydatid cyst REF: Sutton's radiology, 7th edition, volume 1 page 771 Signs of Hydatid cyst: Echinococcus Meniscus/crescent/double arch sign: Floating membrane sign: Spoke wheel/Ca wheel appearance Cumbo sign Water Lilly sign: Camalotte sign: A cyst with an undulating membrane Serpent sign: No calcification in lung", 'role': 'assistant'}]