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?

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

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)_