A crisis in a patient with sickle cell disease is most likely to be cause by

A crisis in sickle cell disease occurs when deoxygenated HbS precipitates, deforming (sickling) red blood cells and .blocking capillaries. Variables that increase the proportion ofHbS in the "deoxy" state tend to increase the probability of a crisis by increasing the extent of sickling. The "deoxy" form of hemoglobin is favored by a decrease in pH (acidosis). An increase in pH (alkalosis), increased oxygen concentration, decreased CO2 concentration, and decreased 2,3-bisphospho­glycerate concentration would all favor the" oxy" form of hemoglobin.