**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 breakdown of sphingomyelin due to enzyme deficiency. **Acid sphingomyelinase** is the enzyme responsible for this process.

**Why the Correct Answer is Right**
The correct answer is the enzyme **acid sphingomyelinase**, which is deficient in Niemann-Pick disease. This enzyme is responsible for breaking down **sphingomyelin** into **ceramide** and **phosphocholine**. Without it, sphingomyelin accumulates in **lysosomes**, leading to cellular dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correspond to the enzyme deficiency in Niemann-Pick disease.
**Option B:** Similarly, this option is incorrect as it is not related to sphingomyelin metabolism.
**Option D:** This option is also incorrect as the enzyme it represents is not involved in the pathogenesis of Niemann-Pick disease.

**Clinical Pearl / High-Yield Fact**
Niemann-Pick disease presents with **hepatosplenomegaly**, **neurological symptoms**, and **developmental delay**. Early diagnosis is crucial for management.

**Correct Answer:** D. Acid sphingomyelinase