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': 'A 50 year old male patient complains of reduced mouth opening. Patient gives h/o RTA 1 week back. A panoramic X-ray was taken using an intensifying screen containing gadolinium and lanthanum. When compared to calcium tungstate screen, the screen used in this patient decreases patient exposure by: \nSelect and explain. \nA. 55%, B. 45%, C. 65%, D. 35%', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Contemporary intensifying screens used in extraoral radiography\xa0use the rare earth elements gadolinium and lanthanum. These rare earth phosphors emit green light on interaction with\xa0X-rays.\xa0Compared with the older calcium tungstate screens, rare earth screens decrease patient exposure by\xa055% in panoramic and cephalometric radiography.\nWhite and Pharoah, Oral radiology, ed 7th, pg-34', 'role': 'assistant'}]