This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Sickle Cell Disease, G6PD deficiency and other Hemolytic Anemia, Subject name: Medicine
[{'content': 'Basic defect in HbS is? \nSelect and explain. \nA. Altered function, B. Altered solubility, C. Altered stability, D. Altered O2 binding capacity', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Basic defect in HbS is altered solubility. A molecule of hemoglobin S (HbS) contains two normal a globin chains and two mutant b globin chains- glutamate at position 6 is substituted with valine. This replaces the polar glutamine residue with a nonpolar valine. The replacement of glutamate by valine generates a sticky patch on the surface of HbS. The sticky patch is present on both oxygenated and deoxygenated HbS but deoxygenated HbS also contains a complementary site for the sticky patch. When HbS is deoxygenated the sticky patch present on its surface binds to the complementary patch on another deoxygenated HbS molecule. Binding of a number of deoxygenated HbS leads to formation of long fibrous polymers of HbS. This stiffens and disto the red cells producing rigid misshaped erythrocytes.', 'role': 'assistant'}]