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: Medicine
[{'content': 'Which of the following region of spine is most commonly affected in rheumatoid ahritis? \nSelect and explain. \nA. Cervical spine, B. Thoracic spine, C. Lumbar spine, D. Sacral spine', 'role': 'user'}, {'content': "Answer: A \nExplanation: Rheumatoid ahritis principally involves atlantoaxial joint of cervical spine. Atlantoaxial involvement of the cervical spine result in compressive myelopathy and neurologic dysfunction. Rheumatoid ahritis does not affect thoracic and lumbar spine except in unusual circumstances. Reference: Harrison's Principles of Internal Medicine 18e chapter 321.", 'role': 'assistant'}]