Lysosomal Storage Disorders
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Direct answer
Lysosomal storage disorders are inherited deficiencies of lysosomal hydrolases (or of the machinery that delivers them) that cause undegraded substrate to accumulate within lysosomes. The group divides into sphingolipidoses, mucopolysaccharidoses, lysosomal glycogen storage (Pompe disease) and trafficking defects such as I-cell disease; almost all are autosomal recessive, with Fabry disease and Hunter syndrome the X-linked exceptions. NEET-PG tests each disorder as a single enzyme-substrate-cell triad, so the topic is pure high-yield recall.
What you must remember
- Gaucher disease — the commonest lysosomal storage disorder: glucocerebrosidase deficiency storing glucocerebroside in macrophages; Gaucher cells have crumpled tissue-paper, PAS-positive cytoplasm, producing hepatosplenomegaly, pancytopenia and Erlenmeyer flask femora; type 1 lacks neurological involvement.
- Niemann-Pick disease types A and B: acid sphingomyelinase deficiency storing sphingomyelin in foam cells; cherry-red macula, hepatosplenomegaly and neurodegeneration in type A.
- Tay-Sachs disease: hexosaminidase A deficiency storing GM2 ganglioside; Ashkenazi Jewish association, cherry-red macula, exaggerated startle, macrocephaly and no hepatosplenomegaly.
- Fabry disease — X-linked: alpha-galactosidase A deficiency storing globotriaosylceramide; angiokeratomas, acroparaesthesia, corneal verticillata, renal failure and left ventricular hypertrophy.
- Mucopolysaccharidoses: Hurler (MPS I, alpha-L-iduronidase) with coarse facies, corneal clouding, dysostosis multiplex and intellectual decline; Hunter (MPS II, iduronate sulfatase) — the X-linked MPS, milder without corneal clouding; Sanfilippo (MPS III) with predominant neurodegeneration; Morquio (MPS IV) with severe skeletal disease and odontoid hypoplasia but normal intellect.
- Pompe disease and I-cell disease: Pompe is lysosomal acid alpha-glucosidase deficiency storing glycogen, with infantile hypotonia and hypertrophic cardiomegaly; I-cell disease is deficient phosphotransferase, so lysosomal enzymes lack the mannose-6-phosphate address and are secreted — high serum enzyme levels, inclusion-filled cells, Hurler-like features with gingival hyperplasia.
- Diagnosis and therapy: enzyme assay on leucocytes or fibroblasts; options include enzyme replacement, stem cell transplantation for some MPS and substrate reduction.
Common confusion
Tay-Sachs versus Niemann-Pick is the classic pair — both show the cherry-red macula, but Tay-Sachs lacks hepatosplenomegaly because ganglioside accumulates in neurons, whereas Niemann-Pick foam cells fill the reticuloendothelial system. The second confusion is Hurler versus Hunter: Hurler is autosomal recessive with corneal clouding, Hunter is X-linked recessive without corneal clouding and runs a milder course. Finally, Gaucher and Niemann-Pick cells are both macrophage-derived — the crumpled tissue-paper look of Gaucher contrasts with the vacuolated foam cell of Niemann-Pick.
Exam-focused takeaway
NEET-PG questions are usually matching exercises: name the enzyme from the substrate or cell, or the syndrome from a vignette — a child with cherry-red macula and startle, a boy with angiokeratomas and renal failure, a coarse-faced child with or without cloudy corneas, or a floppy infant with massive cardiomegaly. Expect one-liners on the two X-linked exceptions, the mannose-6-phosphate defect of I-cell disease, and which disorders are treatable by enzyme replacement. Build an enzyme-substrate-hallmark cell grid for the named disorders and revise it as one unit.
Frequently asked questions
What causes lysosomal storage disorders?
Deficiency of a lysosomal hydrolase — or of the trafficking that delivers enzymes to lysosomes — so substrate accumulates within lysosomes and distends the affected cells.
Which are the two X-linked lysosomal storage disorders?
Fabry disease (alpha-galactosidase A deficiency) and Hunter syndrome or MPS II (iduronate sulfatase deficiency); all the others are autosomal recessive.
How do Tay-Sachs and Niemann-Pick disease differ?
Tay-Sachs lacks organomegaly because GM2 ganglioside from hexosaminidase A deficiency stores in neurons; Niemann-Pick's acid sphingomyelinase deficiency fills foam cells with sphingomyelin, so hepatosplenomegaly accompanies the type A cherry-red macula.
What is the characteristic cell of Gaucher disease?
A macrophage distended by glucocerebroside, with crumpled tissue-paper, PAS-positive cytoplasm.
How do Hurler and Hunter syndromes differ?
Hurler (MPS I) is autosomal recessive with corneal clouding and severe cognitive decline; Hunter (MPS II) is X-linked recessive, milder, without corneal clouding.
What is the defect in I-cell disease?
Absent N-acetylglucosaminyl-1-phosphotransferase, so lysosomal enzymes lack the mannose-6-phosphate address and are secreted — plasma enzyme activity rises while lysosomes fill with inclusions.