**Core Concept**
Niemann-Pick disease is a group of genetic disorders caused by the accumulation of sphingomyelin, a type of lipid, within cells due to a deficiency of the enzyme sphingomyelinase. This enzyme is responsible for breaking down sphingomyelin into ceramide and phosphocholine.

**Why the Correct Answer is Right**
The accumulation of sphingomyelin in cells leads to cellular dysfunction and death, resulting in the characteristic symptoms of Niemann-Pick disease, including neurodegeneration, hepatosplenomegaly, and respiratory problems. The deficiency of sphingomyelinase is due to mutations in the SMPD1 gene, which encodes for the enzyme. The enzyme's activity is crucial for the normal breakdown of sphingomyelin, and its deficiency leads to the accumulation of this lipid in cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Niemann-Pick disease is not primarily caused by a disturbance in fatty acid metabolism. While sphingomyelin is a type of lipid, the disease is specifically related to the breakdown of this particular lipid, not fatty acid metabolism in general.
* **Option B:** This option is incorrect because Gaucher disease, not Niemann-Pick disease, is caused by a deficiency of the enzyme glucocerebrosidase, which breaks down glucocerebroside. Gaucher disease is another lysosomal storage disorder, but it is distinct from Niemann-Pick disease.
* **Option C:** This option is incorrect because Tay-Sachs disease, not Niemann-Pick disease, is caused by a deficiency of the enzyme hexosaminidase A, which breaks down GM2 ganglioside. Tay-Sachs disease is another lysosomal storage disorder, but it is distinct from Niemann-Pick disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Niemann-Pick disease is caused by a deficiency of the enzyme sphingomyelinase, which breaks down sphingomyelin. This enzyme deficiency leads to the accumulation of sphingomyelin in cells, resulting in the characteristic symptoms of the disease.

**Correct Answer:** D.