A child presents with neurodegeneration and a cherry-red spot on fundus examination. Enzyme assay reveals hexosaminidase A deficiency. What is the most likely diagnosis?

Explanation

## Correct Answer: C. GM2 gangliosidosis GM2 gangliosidosis is a lysosomal storage disorder caused by deficiency of **hexosaminidase A (Hex-A)**, the enzyme responsible for degrading GM2 gangliosides. The classic presentation combines progressive neurodegeneration with a pathognomonic **cherry-red spot on the macula** during fundoscopic examination. The cherry-red spot results from accumulation of GM2 gangliosides in retinal ganglion cells, creating a pale fovea surrounded by red retina. Tay-Sachs disease (infantile GM2 gangliosidosis) is the most common form in Indian populations, presenting in infants aged 3–6 months with developmental regression, hypotonia, seizures, and blindness. The enzyme assay demonstrating Hex-A deficiency is diagnostic—Hex-B remains normal. Inheritance is autosomal recessive. Early recognition is critical for genetic counseling and prenatal diagnosis in subsequent pregnancies, as the condition is invariably fatal by age 3–4 years without curative therapy. The combination of neurodegeneration + cherry-red spot is virtually pathognomonic for GM2 gangliosidosis among lysosomal storage disorders. ## Why the other options are wrong **A. GM1 gangliosidosis** — GM1 gangliosidosis results from **β-galactosidase deficiency**, not Hex-A deficiency. While it also presents with neurodegeneration and can show a cherry-red spot, the enzyme assay would reveal β-galactosidase deficiency. GM1 also features hepatosplenomegaly and skeletal dysplasia more prominently than GM2. The discriminating enzyme finding rules this out. **B. Niemann-Pick disease** — Niemann-Pick disease results from **sphingomyelinase deficiency**, causing sphingomyelin accumulation. While it presents with neurodegeneration and hepatosplenomegaly, the cherry-red spot is less characteristic (and when present, less prominent). The enzyme assay would show sphingomyelinase deficiency, not Hex-A deficiency. This is a common NBE trap pairing multiple lysosomal disorders. **D. Gaucher disease** — Gaucher disease results from **glucocerebrosidase deficiency**, causing glucocerebroside accumulation. The hallmark finding is hepatosplenomegaly with 'Gaucher cells' on bone marrow examination, not neurodegeneration and cherry-red spot. Type 1 (non-neuropathic) is most common in India. The enzyme assay would show glucocerebrosidase deficiency, making this easily excluded. ## High-Yield Facts - **Hexosaminidase A deficiency** → GM2 gangliosidosis (Tay-Sachs disease); autosomal recessive inheritance. - **Cherry-red spot on macula** is pathognomonic for GM2 gangliosidosis; caused by GM2 ganglioside accumulation in retinal ganglion cells. - **Tay-Sachs onset** typically 3–6 months of age with developmental regression, hypotonia, seizures, and blindness; fatal by age 3–4 years. - **Enzyme assay discriminator**: Hex-A deficiency with normal Hex-B confirms GM2 gangliosidosis; other lysosomal disorders show different enzyme patterns. - **Indian epidemiology**: Tay-Sachs is rare in Indian populations compared to Ashkenazi Jews, but Niemann-Pick and Gaucher are more prevalent—enzyme assay is critical. ## Mnemonics **CHERRY-RED SPOT LYSOSOMAL DISORDERS** **C**entral retinal artery occlusion (not lysosomal), **GM2** gangliosidosis (Hex-A deficiency), **Sialidosis**, **Galactosialidosis**. In pediatric neurodegeneration, GM2 is the most common cause of cherry-red spot. **GM2 GANGLIOSIDOSIS VARIANTS (Hex-A deficiency)** **Type 1 (Tay-Sachs)**: Infantile, 3–6 months, fatal by 3–4 years. **Type 2 (Sandhoff)**: Hex-A AND Hex-B deficiency (different gene). **Type 3 (Juvenile)**: Later onset, slower progression. Enzyme assay distinguishes them. ## NBE Trap NBE pairs multiple lysosomal storage disorders (GM1, Niemann-Pick, Gaucher) with neurodegeneration to test whether students rely solely on clinical presentation rather than the discriminating enzyme finding. The enzyme assay is the key differentiator—students who guess based on "cherry-red spot" alone may incorrectly choose GM1 gangliosidosis. ## Clinical Pearl In Indian pediatric practice, when a parent reports an infant with normal development until 3–6 months followed by rapid regression and loss of vision, GM2 gangliosidosis must be ruled out immediately with enzyme assay. Early diagnosis enables genetic counseling and prenatal diagnosis in future pregnancies, which is critical in resource-limited settings where curative therapies are unavailable. _Reference: Robbins Ch. 5 (Genetic and Pediatric Diseases); OP Ghai Essentials of Pediatrics Ch. 10 (Metabolic and Genetic Disorders)_