A child demonstrates progressive neurological decline and a cherry-red spot-on fundus examination. Enzyme analysis reveals a deficiency in hexosaminidase A. What is the most likely diagnosis?

Explanation

## Correct Answer: B. GM2 gangliosidosis GM2 gangliosidosis is a lysosomal storage disorder caused by deficiency of **hexosaminidase A (Hex-A)**, the enzyme responsible for degrading GM2 gangliosides, predominantly in the brain. The combination of progressive neurological decline and a **cherry-red spot on fundus examination** is pathognomonic for GM2 gangliosidosis (Tay-Sachs disease being the classic infantile form). The cherry-red spot appears because gangliosides accumulate in the retina, creating a white opaque appearance around the macula, while the fovea (which lacks gangliosides) appears red due to underlying choroidal blood vessels. Hex-A deficiency leads to accumulation of GM2 gangliosides in neurons, causing progressive neuronal dysfunction, developmental regression, seizures, blindness, and death typically by age 3–5 years in the infantile form. The enzyme deficiency is confirmed by enzyme assay showing absent or severely reduced Hex-A activity. This is the most common lysosomal storage disorder in Ashkenazi Jewish populations, though cases occur across all ethnic groups in India. The diagnosis is confirmed by demonstrating reduced Hex-A activity in serum, leukocytes, or fibroblasts, often with normal or elevated hexosaminidase B (Hex-B) activity. ## Why the other options are wrong **A. Niemann-Pick disease** — Niemann-Pick disease results from **sphingomyelinase deficiency**, not Hex-A deficiency. While it also presents with neurological decline and hepatosplenomegaly, it does NOT produce a cherry-red spot on fundoscopy. The characteristic finding is a 'cherry-red spot' in the macula in some variants, but the enzyme defect is entirely different. NBE may trap students who confuse lysosomal storage disorders without carefully matching enzyme deficiency to clinical findings. **C. GM1 gangliosidosis** — GM1 gangliosidosis results from **β-galactosidase deficiency**, not Hex-A deficiency. Although it also causes progressive neurological decline and can present with a cherry-red spot, the enzyme defect is distinct. GM1 gangliosidosis typically presents with coarse facial features, skeletal dysplasia (especially in infantile form), and hepatosplenomegaly more prominently than GM2. The question explicitly states Hex-A deficiency, which rules out GM1 entirely. **D. Gaucher disease** — Gaucher disease is caused by **glucocerebrosidase deficiency**, leading to accumulation of glucocerebroside in macrophages (Gaucher cells). It presents with hepatosplenomegaly, bone involvement, and anemia, but does NOT cause a cherry-red spot or the acute neurological decline seen here. Type 1 (non-neuropathic) is most common in India; neurological forms are rare. The enzyme defect and clinical presentation are fundamentally different from GM2 gangliosidosis. ## High-Yield Facts - **Hex-A deficiency** → GM2 gangliosidosis (Tay-Sachs disease); **cherry-red spot + progressive neurological decline** is pathognomonic. - **Cherry-red spot** appears due to ganglioside accumulation in retina creating white opaque appearance around macula; fovea remains red (normal choroid visible). - **Infantile GM2 gangliosidosis** presents at 3–6 months with developmental regression, seizures, blindness, and death by age 3–5 years. - **Enzyme diagnosis**: Hex-A activity is absent or severely reduced; Hex-B activity is normal or elevated (helps differentiate from Sandhoff disease where both are deficient). - **Lysosomal storage disorder classification**: GM2 gangliosidosis is a **sphingolipidosis** (ganglioside accumulation), distinct from Niemann-Pick (sphingomyelinosis) and Gaucher (glucocerebrosidosis). ## Mnemonics **Cherry-Red Spot Disorders (CREST)** **C**entral retinal artery occlusion, **R**etinitis pigmentosa, **E**mboli, **S**torage diseases (Tay-Sachs/GM2, Sialidosis, GM1), **T**oxoplasmosis. In pediatric progressive neurological decline, think **Tay-Sachs (GM2 gangliosidosis)** first. **Hex-A vs Hex-B (Tay-Sachs vs Sandhoff)** **Tay-Sachs**: Hex-A ↓↓, Hex-B normal. **Sandhoff**: Both Hex-A and Hex-B ↓↓. Both present identically clinically, but enzyme pattern differentiates them. Use this when enzyme results are given. ## NBE Trap NBE pairs "cherry-red spot" with multiple lysosomal storage disorders (GM1, Niemann-Pick, Sialidosis) to trap students who recognize the sign but fail to match it specifically to the enzyme deficiency stated in the question. The discriminator is **Hex-A deficiency**, which is pathognomonic for GM2 gangliosidosis. ## Clinical Pearl In Indian pediatric practice, when a child presents with regression, seizures, and blindness by 6–12 months with a cherry-red spot, GM2 gangliosidosis (Tay-Sachs) must be ruled out urgently via Hex-A enzyme assay. Prenatal diagnosis via chorionic villus sampling is available for at-risk families, and genetic counseling is critical given the autosomal recessive inheritance and poor prognosis. _Reference: Robbins Ch. 5 (Genetic and Pediatric Diseases); OP Ghai Pediatrics Ch. 8 (Inborn Errors of Metabolism)_