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': 'Radiation caries occurs due to all of the following except: \nSelect and explain. \nA. Bone defects, B. Fibrosis of salivary glands, C. Decreased flow of saliva, D. Low pH of saliva', 'role': 'user'}, {'content': "Answer: A \nExplanation: Radiation Caries\nRadiation caries is a rampant form of dental decay that may occur in individuals who receive a course of radiotherapy that includes exposure of the salivary glands. Patients receiving radiation therapy to oral structures have an increase\xa0in Streptococcus mutans, Lactobacillus, and Candida.\nCaries results from changes in the salivary glands and saliva, including reduced flow, decreased pH, reduced buffering capacity, increased viscosity, and altered flora.\xa0\xa0\nThe residual saliva in individuals with xerostomia also has a low concentration of Ca2+ ion; this results in greater solubility of tooth structure and reduced remineralization. Because of the reduced or absent cleansing action of normal saliva, debris accumulates quickly.\xa0\nThere is also growing evidence that radiation has direct effects on teeth that make them more prone to breakdown with flaking of enamel, particularly in areas of occlusal loading or stress,\xa0such as at the incisal, cuspal, and cervical regions of teeth. The destruction is seen with doses greater than 30 Gy and is pronounced when the teeth receive more than 60 Gy.\nReference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 22", 'role': 'assistant'}]