Our cells have organelles called lysosomes, which are fundamentally packets of up to 50 destructive enzymes that can be hurled at whatever needs degrading - be it an old cellular component or a foreign microorganism. From time to time, a person can inherit a defective gene for one of the enzymes, and the result is predictable: you can't break down that substance. As a result, the substance accumulates in various tissues, which it can damage. This is what is meant by "lysosomal storage disease", although "storage" seems a bit polite - the substance clogs up the works despite the body's best efforts.
Let's concentrate on one semi-famous example: Gaucher's disease. This is caused by a deficiency of an enzyme called glucocerebrosidase, which is an enzyme that lysozymes use to degrade a lipid called (funnily enough) glucocerebroside. As a result, this fatty substance accumulates in organs like the liver, spleen, bone marrow, brain and lungs - and hence the condition finds its way onto your lists of splenomegaly causes.
Lysosomal storage diseases can be subdivided into four large groups, which I'll mention just for completeness' sake. There is more than one condition under each heading, of course:
- Lipid storage diseases (e.g. Gaucher's disease)
- Mucopolysaccharidoses - this weird name indicates only an inability to break down one of the glycosaminoglycans
- Glycoprotein storage disorders