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': 'Human leukocyte antigen ( HLA) DR 4 occurs in about 70% of patients with rheumatoid arthritis HLA R4 encoded in the major histocompatibility complex (MEW) region on - \nSelect and explain. \nA. Chromosome 6, B. Chromosome 9, C. Chromosome 18, D. Choromosome 22', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]