**Question:** Lysosomal accumulation of sphingomyeline is seen in-

A. Tay-Sachs disease
B. Niemann-Pick disease
C. Krabbe disease
D. Fabry disease

**Core Concept:** Lysosomes are membrane-bound organelles found in eukaryotic cells, which function in the hydrolysis of cellular waste products and recycling of cellular components. Sphingomyelin is a phospholipid component of cellular membranes, and its accumulation within lysosomes can lead to lysosomal storage disorders.

**Core Concept (Expanded):** Sphingomyelin is a sphingolipid, a type of phospholipid, that forms the main component of cell membranes, particularly in the myelin sheath of neurons. Sphingomyelin is synthesized and dephosphorylated by various enzymes, ultimately leading to its incorporation into the cell membrane. The lysosomal enzymes that degrade sphingomyelin are defective in lysosomal storage disorders.

**Why the Correct Answer is Right:** In the correct answer, lysosomal accumulation of sphingomyelin is seen in Niemann-Pick disease. This is a group of lysosomal storage disorders characterized by the accumulation of sphingolipids in various tissues due to defective lysosomal enzymes such as acid sphingomyelinase. The deficiency in this enzyme leads to the accumulation of sphingomyelin and cholesterol, causing damage to organs like the liver, spleen, and lungs.

**Why Each Wrong Option is Incorrect:**

A. Tay-Sachs disease is a different lysosomal storage disorder caused by the deficiency of the enzyme α-galactosidase A, leading to the accumulation of gangliosides (a type of glycolipid) rather than sphingomyelin.

B. Niemann-Pick type C disease is another lysosomal storage disorder caused by a deficiency in sphingomyelin synthase, which results in an imbalance between sphingomyelin and cholesterol synthesis.

C. Krabbe disease is a lysosomal storage disorder caused by the deficiency of galactocerebrosidase, which leads to the accumulation of galactosylceramides, not sphingomyelin.

D. Fabry disease is caused by the deficiency of α-galactosidase, leading to the accumulation of globotriaosylceramide (GL-3), not sphingomyelin.

E. Niemann-Pick type A/B disease is related to the accumulation of cholesterol, not sphingomyelin.

**Clinical Pearls:**

1. Niemann-Pick disease is a prime example of a lysosomal storage disorder that affects the liver, spleen, and lungs, leading to progressive organ dysfunction and failure.
2. The distinction between different subtypes of Niemann-Pick disease depends on the specific enzyme deficiency and the accumulation of different lysosomal storage molecules.
3. Clinical presentation, imaging findings, and response to treatment can help differentiate between different lysosomal storage disorders.
4. Familiarity with the different lysosomal storage disorders can help identify potential cases in clinical practice and guide appropriate management strategies.