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Elevated cellular pH
16%
34/210
Elevated 2,3-diphosphoglycerate
61%
129/210
Elevated arterial oxygen concentration
3%
6/210
Elevated hemoglobin F levels
10%
21/210
Travel from high to low elevation geographical area
9%
19/210
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This patient is presenting with a sickle cell crisis, which can be precipitated in part by high levels of 2,3-diphosphoglycerate (2,3-DPG). Sickle cell anemia is an autosomal recessive disorder due to a mutation in the hemoglobin ß-chain. In the disease, Hemoglobin S (HbS) tetramers can polymerize, thereby causing sickling of deoxygenated red blood cells (RBCs). For sickling to occur in the systemic vasculature, greater than 90% HbS is required. One notable exception is in the renal papilla where oxygen tension is low enough to induce sickling even when HbS levels are lower than 90%. Noguchi et al. discuss the pathophysiology of sickling. The major event that initiates sickling is the polymerization of HbS. The extent of polymerization is controlled by several factors, including the relative composition of the types of hemoglobin within the cell (e.g., HbS, HbF, etc.), mean corpuscular hemoglobin concentration, and oxygen saturation. Additional factors, including intracellular pH and DPG concentration play a minor, but still important role. Other abnormalities may also play a role, including abnormalities in RBC membranes, or changes in ionic concentrations or fluid volume status. Manwani and Frenette review another element in the pathophysiology of the vasooclusive cascade. In addition to the reasons discussed above, sickled hemoglobin can cause damage to RBC membranes directly. The sickled, mutated protein can undergo auto-oxidation, which can lead to precipitation on the inner surface of the RBC membrane, and can cause damage to the RBC. This can change the propensity of the RBC to adhere to vasculature surfaces, precipitating vasooclusive crises. Figure A is a blood smear showing sickled RBCs, the characteristic hematologic finding of sickle cell anemia. Illustration A is a diagram of a normal RBC in cross section, as well as a sickled RBC in cross section. In the sickled RBC, note the polymerization of hemoglobin in the affected cell. Incorrect answers: Answers 1, 3-5: each of these answers would lead to a decreased, not increased, propensity for red blood cells to sickle.
3.1
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