Relation between multiple hierarchical taxonomies linked to a single entity
Taxonomy -&gt; TaxonomyTerm
TaxonomyTerm -&gt; Subject
Subject -&gt; Topic
Topic -&gt; Question


Each of these lines is a 1 to many relationship, e.g. A single Taxonomy can have 1 or more TaxonomyTerms, and these terms can be related to multiple Taxonomies. I think you mostly have it but I'm not seeing a need to link one TaxonomyTerm directly with another.