A 22-year-old woman delivers an apparently healthy female infant after an uncomplicated pregnancy. By 4 years of age, the girl has progressive, severe neurologic deterioration. Physical examination shows marked hepatosplenomegaly. A bone marrow biopsy specimen shows numerous foamy vacuolated macrophages. Analysis of which of the following factors is most likely to aid in the diagnosis of this condition?
Level of a1-antitrypsin in the serum Level of glucose-6-phosphatase in hepatocytes Level of sphingomyelinase in splenic macrophages Number of LDL receptors on hepatocytes