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: Dental
[{'content': 'During a cephalometric examination SNA is decreased. Which of the following is the most probable reason? \nSelect and explain. \nA. Maxilla is retrognathic to anterior cranial base, B. Mandible is prognathic to anterior cranial base, C. Maxilla is prognathic to anterior cranial base, D. Mandible is retrognathic to anterior cranial base', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Maxilla (SNA)\nPoints A and B are regarded as the anterior limits of the apical bases of the maxilla and mandible, respectively.\nTherefore, to determine whether the maxilla is positioned anteriorly or posteriorly to the cranial base, the angle SNA is noted.\nThe angle SNA is formed by sella-nasion and nasion to point-A. The mean SNA reading is 82 degrees, however, if the angular reading is greater than 82 degrees, it would indicate a relative forward positioning of the maxilla. Conversely, if the reading is less than 82 degrees, it would indicate a relative backward or recessive location of the maxilla.', 'role': 'assistant'}]