A neonate presents with seizures and is found to have a cherry red spot on fundus examination. Enzyme assay reveals deficiency of hexosaminidase A. Which of the following substances is most likely to be accumulated in this patient?

Explanation

## Correct Answer: D. GM2 ganglioside Tay-Sachs disease is caused by deficiency of **hexosaminidase A (Hex-A)**, an enzyme responsible for degrading **GM2 ganglioside**. The cherry red spot on fundus examination is a pathognomonic sign—it represents retinal whitening due to ganglioside accumulation in retinal neurons, with the fovea appearing red because it lacks ganglioside-laden cells. Hex-A catalyzes the removal of N-acetylgalactosamine from GM2 ganglioside; without this enzyme, GM2 accumulates in neurons, lysosomes, and retinal cells. The disease presents in infants (typically 3–6 months) with developmental regression, seizures, hypotonia, and blindness. GM2 ganglioside is a **sphingolipid** with a complex structure containing a ceramide backbone, oligosaccharide chain, and sialic acid residues. Accumulation in the CNS causes progressive neuronal dysfunction. This is distinct from other lysosomal storage disorders: GM1 gangliosidosis (GM1 accumulation, β-galactosidase deficiency), Gaucher disease (glucocerebroside, glucocerebrosidase deficiency), and Niemann-Pick disease (sphingomyelin, sphingomyelinase deficiency). The clinical triad of cherry red spot + seizures + developmental regression in an infant is virtually diagnostic of Tay-Sachs in the Indian population, where consanguinity increases carrier frequency. ## Why the other options are wrong **A. Glucocerebroside** — Glucocerebroside accumulates in **Gaucher disease** (β-glucocerebrosidase deficiency), not Tay-Sachs. Gaucher disease presents with hepatosplenomegaly and bone involvement, NOT cherry red spot or acute neonatal seizures. This is a common trap—students confuse lysosomal storage disorders by substrate rather than enzyme. **B. Sphingomyelin** — Sphingomyelin accumulates in **Niemann-Pick disease** (sphingomyelinase deficiency), which presents with hepatosplenomegaly and pulmonary involvement. While Niemann-Pick can cause neurological symptoms, it does NOT produce the characteristic cherry red spot seen in Tay-Sachs. The fundoscopic finding is the key discriminator here. **C. GM1 ganglioside** — GM1 gangliosidosis results from **β-galactosidase deficiency**, not Hex-A deficiency. Although GM1 gangliosidosis also presents with infantile seizures and developmental regression, it typically includes **coarse facial features and skeletal abnormalities** (unlike Tay-Sachs), and the cherry red spot is less characteristic. Hex-A deficiency specifically causes GM2 accumulation. ## High-Yield Facts - **Tay-Sachs disease** = Hexosaminidase A deficiency → GM2 ganglioside accumulation in CNS and retina. - **Cherry red spot** on fundus is pathognomonic for Tay-Sachs; represents retinal whitening with spared fovea due to ganglioside deposition. - **Infantile presentation**: seizures, developmental regression, blindness, hypotonia by 3–6 months; death by age 3–4 years. - **Autosomal recessive** inheritance; higher carrier frequency in Ashkenazi Jewish and some Indian populations due to consanguinity. - **Hex-A enzyme** cleaves N-acetylgalactosamine from GM2 ganglioside; deficiency → lysosomal accumulation in neurons. - **Prenatal diagnosis** via amniocentesis (low Hex-A activity) and **genetic counseling** are critical in high-risk families in India. ## Mnemonics **Cherry Red Spot = Tay-Sachs (CRS-TS)** **C**herry **R**ed **S**pot → **T**ay-**S**achs. When you see cherry red spot + seizures + regression in an infant, think Hex-A deficiency and GM2 accumulation immediately. **Ganglioside Storage Diseases (GSDs)** **GM2** (Tay-Sachs, Hex-A) | **GM1** (β-galactosidase) | **Sphingomyelin** (Niemann-Pick) | **Glucocerebroside** (Gaucher). Match enzyme deficiency to substrate, not just the disease name. ## NBE Trap NBE pairs "cherry red spot" with multiple gangliosidoses to trap students who don't recall that Tay-Sachs (Hex-A deficiency → GM2) is the ONLY one with this pathognomonic fundoscopic sign. Confusing GM1 gangliosidosis (which has coarse features, not cherry red spot) is the most common error. ## Clinical Pearl In Indian pediatric practice, Tay-Sachs should be suspected in any neonate with seizures + developmental regression + cherry red spot, especially in families with consanguinity. Early enzyme assay (Hex-A activity in leukocytes or fibroblasts) confirms diagnosis; genetic counseling and prenatal diagnosis in subsequent pregnancies are essential given the autosomal recessive inheritance and high carrier frequency in certain Indian communities. _Reference: Robbins Ch. 5 (Lysosomal Storage Diseases); Harrison Ch. 349 (Lysosomal Disorders); KD Tripathi Ch. 37 (Lipid Metabolism & Storage Disorders)_