Characteristic of intravascular hemolysis

Markedly decreased blood levels of which one of the listed substances are most characteristic of intravascular hemolysis?

A. Alkaline phosphatase 
B. Bilirubin
C. Haptoglobin 
D. Lactate dehydrogenase

The answer is c. Destruction of red cells (hemolysis) may occur within the vascular compartment (intravascular hemolysis) or within the mononuclear-phagocyte system (extravascular hemolysis). In both cases, the hemolysis leads to anemia, and the breakdown of hemoglobin leads to jaundice due to increased indirect bilirubin. Intravascular hemolysis releases hemoglobin into the blood (hemoglobinemia); this hemoglobin then binds to haptoglobin. When haptoglobin levels are depleted, free hemoglobin is oxidized to methemoglobin, and then both hemoglobin and methemoglobin are secreted into the urine (hemoglobinuria and methemoglobinuria). Within the renal tubular epithelial cells, hemoglobin is reabsorbed and hemosiderin is formed; when these cells are shed into the urine, hemosiderinuria results. Since extravascular hemolysis does not occur within the vascular compartment, hemoglobin- emia, hemoglobinuria, methemoglobinuria, and hemosiderinuria do not occur. The breakdown of the red cells within the phagocytic cells causes anemia and jaundice, just as with intravascular hemolysis, and, since hemoglobin escapes into the blood from the phagocytic cells, plasma haptoglobin levels are also reduced. Unlike the case with intravascular hemolysis, the erythrophagocytosis causes hypertrophy and hyperplasia of the mononuclear phagocytic system, which in turn may lead to splenomegaly.