Lipid Storage — Niemann-Pick Foam Cells

A 2-year-old boy of Ashkenazi Jewish descent presents with failure to thrive, massive hepatosplenomegaly, and developmental regression. Bone marrow aspirate shows large foamy histiocytes. The pathologic process marked **D** in the diagram—sphingomyelinase deficiency—is responsible for the accumulation of substrate material in these cells. Which of the following best describes the biochemical consequence and the primary clinical phenotype associated with this enzymatic defect in Niemann-Pick disease type A?