**Core Concept**
Gaucher's disease is a genetic disorder caused by a deficiency of the enzyme glucocerebrosidase, leading to the accumulation of its substrate, glucocerebroside, within cells. This accumulation results in the formation of Gaucher cells, which are characteristic of the disease.

**Why the Correct Answer is Right**
The correct answer is glucocerebroside. Glucocerebrosidase is a lysosomal enzyme responsible for breaking down glucocerebroside into glucose and ceramide. Without sufficient glucocerebrosidase activity, glucocerebroside accumulates within lysosomes of cells, particularly in macrophages and spleen cells. This accumulation disrupts normal cellular function and leads to the characteristic features of Gaucher's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the substrate accumulation in Gaucher's disease. Sphingomyelin is the substrate that accumulates in Niemann-Pick disease, not Gaucher's disease.
**Option B:** This option is incorrect because it does not accurately describe the substrate accumulation in Gaucher's disease. Glycogen is the storage form of glucose, but it is not the substrate that accumulates in Gaucher's disease.
**Option C:** This option is incorrect because it does not accurately describe the substrate accumulation in Gaucher's disease. Ceramide is a breakdown product of glucocerebroside, but it is not the substrate that accumulates in Gaucher's disease.

**Clinical Pearl / High-Yield Fact**
Gaucher's disease is an autosomal recessive disorder, meaning that a person must inherit two defective copies of the glucocerebrosidase gene (one from each parent) to develop the disease. This highlights the importance of genetic counseling for families with a history of Gaucher's disease.

**Correct Answer: D. Glucocerebroside**