A child presents with mental retardation, bone pain, and inability to walk. On funduscopic examination, a cherry red spot is seen. There is no organomegaly. What is the most likely diagnosis?

Explanation

## Correct Answer: C. Tay-Sachs disease Tay-Sachs disease is a **lysosomal storage disorder** caused by deficiency of the enzyme **hexosaminidase A (Hex-A)**, leading to accumulation of **GM2 gangliosides** in the CNS and retina. The clinical triad presented—mental retardation, bone pain, and inability to walk—reflects progressive neurodegeneration. The **cherry red spot** on fundoscopy is pathognomonic: it represents a pale, swollen macula surrounded by retinal hemorrhages due to ganglioside accumulation in retinal neurons, with the fovea appearing red against the pale background. Notably, **there is no hepatosplenomegaly**, which is a key discriminator from other storage diseases. The disease typically presents in infancy (3–6 months) with developmental regression, seizures, blindness, and death by age 3–4 years. In India, Tay-Sachs is less common than in Ashkenazi Jewish populations but remains an important differential in any child with the cherry red spot sign. The diagnosis is confirmed by demonstrating **reduced Hex-A activity** in leukocytes or fibroblasts and elevated GM2 gangliosides in urine. No specific cure exists; management is supportive. ## Why the other options are wrong **A. Niemann-Pick disease** — Niemann-Pick disease (sphingomyelinase deficiency) does present with mental retardation and neurodegeneration, but it is characterized by **marked hepatosplenomegaly** and vertical supranuclear gaze palsy—neither of which is mentioned in this case. While cherry red spots can occur, they are less typical than in Tay-Sachs. The absence of organomegaly rules this out. **B. Gaucher disease** — Gaucher disease (glucocerebrosidase deficiency) causes bone pain and neurological symptoms in Type 3 (chronic neuronopathic form), but it is dominated by **massive hepatosplenomegaly** and bone marrow infiltration with Gaucher cells. Cherry red spots are not a feature. The clinical presentation here lacks the characteristic organomegaly and bone marrow involvement. **D. Hurler syndrome** — Hurler syndrome (α-L-iduronidase deficiency, MPS I) causes intellectual disability and skeletal abnormalities, but it presents with **coarse facial features, corneal clouding, hepatosplenomegaly, and cardiac involvement**—none of which are mentioned. Cherry red spots are not typical. The clinical picture here is purely neurological without the systemic features of mucopolysaccharidosis. ## High-Yield Facts - **Cherry red spot** on fundoscopy is pathognomonic for Tay-Sachs disease and represents ganglioside accumulation in the macula. - **Hexosaminidase A deficiency** is the enzyme defect in Tay-Sachs; diagnosis confirmed by leukocyte enzyme assay. - **Absence of hepatosplenomegaly** is a key feature that distinguishes Tay-Sachs from Niemann-Pick and Gaucher disease. - **GM2 ganglioside** accumulates in neurons and retinal cells, causing progressive neurodegeneration and blindness. - **Infantile form** presents at 3–6 months with developmental regression, seizures, and death by age 3–4 years. - **No cure exists**; management is supportive care and genetic counseling for carrier detection in families. ## Mnemonics **Cherry Red Spot = Tay-Sachs** When you see **cherry red spot** on fundoscopy in a child with neurodegeneration, think **Tay-Sachs first**. The red fovea against pale retina is the visual signature of GM2 ganglioside accumulation. **Storage Disease Organomegaly Rule** **No spleen/liver = Tay-Sachs or Metachromatic Leukodystrophy**; **Big spleen/liver = Niemann-Pick, Gaucher, or MPS**. This single clinical sign rapidly narrows the differential. ## NBE Trap NBE pairs "bone pain" with Gaucher disease (which is classic for bone involvement) to lure students away from Tay-Sachs. However, bone pain in Tay-Sachs reflects spasticity and neurological involvement, not bone marrow infiltration—and the absence of hepatosplenomegaly is the key discriminator. ## Clinical Pearl In Indian pediatric practice, while Tay-Sachs is rare compared to Ashkenazi Jewish populations, any infant presenting with the **cherry red spot sign** should trigger immediate Tay-Sachs screening via Hex-A enzyme assay. Early genetic counseling prevents recurrence in families, particularly important in communities with higher carrier frequencies. _Reference: Robbins Ch. 5 (Genetic Disorders); Harrison Ch. 356 (Lysosomal Storage Diseases); KD Tripathi Ch. 37 (Lipid Metabolism)_