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: Pathology
[{'content': 'Sickle cell trait patient do not have manifestations as that of Sickle cell disease, because- \nSelect and explain. \nA. 50% HbS is required for occurrence of sickling, B. HbA prevents sickling, C. 50% sickles, D. HbA prevents polymerization of Hbs', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Sickle cell disease occurs in following forms\n\n\nHeterozygous state, i.e., sickle cell trait.\nHomozygous state, i.e., sickle cell anemia.\n\n\nThe most important factor for sickling is the amount of HbS in the cell.\nIn sickle cell trait (heterozygous state), only about 40% of the hemoglobin is HbS, this is insufficient to produce sickling manifestations.\nIn homozygous state, almost all the hemoglobin in the red cell is HbS, therefore they undergo sickling.', 'role': 'assistant'}]