**Core Concept**
The child's presentation of hepatosplenomegaly, lymphadenopathy, psychomotor retardation, and characteristic fundus examination findings, along with bony defects, suggests a lysosomal storage disorder. This group of disorders is caused by the deficiency of specific enzymes involved in the degradation of cellular waste products.

**Why the Correct Answer is Right**
The child's symptoms are consistent with Histiocytosis-Lymphadenopathy Syndrome, also known as Niemann-Pick disease type B. This condition is caused by a deficiency of sphingomyelinase, an enzyme that breaks down sphingomyelin in the lysosomes. The accumulation of sphingomyelin in cells leads to cellular dysfunction and the characteristic clinical and pathological findings seen in this disease. The impaired voluntary rapid eye movements and cheery red spot on fundus examination are particularly suggestive of Niemann-Pick disease type B. Electron microscopy findings would show lysosomal distension due to sphingomyelin accumulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Hexosaminidase A deficiency leads to Tay-Sachs disease, a different lysosomal storage disorder characterized by the accumulation of GM2 gangliosides in neurons, resulting in severe neurodegeneration.

**Option B:** Alpha-galactosidase A deficiency is associated with Fabry disease, a lysosomal storage disorder that affects the breakdown of globotriaosylceramide, leading to systemic symptoms including pain, renal failure, and cardiovascular disease.

**Option C:** Glucocerebrosidase deficiency leads to Gaucher disease, a lysosomal storage disorder characterized by the accumulation of glucocerebroside in macrophages, resulting in hepatosplenomegaly, anemia, and bone marrow failure.

**Clinical Pearl / High-Yield Fact**
Niemann-Pick disease type B can be distinguished from other lysosomal storage disorders by the presence of hepatosplenomegaly, lymphadenopathy, and the characteristic fundus examination findings, including the cheery red spot.

**Correct Answer:**
✓ Correct Answer: D. Sphingomyelinase.