**Core Concept**
Niemann-Pick disease is a group of **lysosomal storage disorders** characterized by the accumulation of **sphingomyelin** in cells. The underlying defect involves the **metabolism of sphingolipids**.

**Why the Correct Answer is Right**
The correct answer is related to the enzyme responsible for breaking down **sphingomyelin**. In Niemann-Pick disease, there is a deficiency of **acid sphingomyelinase**, which is necessary for the degradation of sphingomyelin in lysosomes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to sphingomyelin metabolism.
**Option B:** This option is also incorrect as it is not involved in the breakdown of sphingomyelin.
**Option C:** Similarly, this option is incorrect because it is not the enzyme deficient in Niemann-Pick disease.

**Clinical Pearl / High-Yield Fact**
Niemann-Pick disease presents with **hepatosplenomegaly**, **neurological symptoms**, and **characteristic cherry-red spots** on the macula.

**Correct Answer:** D. Acid sphingomyelinase