Tough is a synonym of Dense. Dense is related to Hard. Hard is related to Description. 
Vehicle is related to Account. Credit is a type of Account. Credit is a different concept from Debt. 
Tough <--Synonym--> Dense <--RelatedTo--> Hard <--RelatedTo--> Description 
Vehicle <--RelatedTo--> Account <--IsA-- Credit <--DistinctFrom--> Debt 