Desk is related to Class. Agriculture is a type of Class. 
Clock is a type of Machine. Mechanic is etymologically related to Machine. Mechanic is related to Doctor. Doctor is a different concept from Patient. 
Desk <--RelatedTo--> Class <--IsA-- Agriculture 
Clock --IsA--> Machine <--EtymologicallyRelatedTo-- Mechanic <--RelatedTo--> Doctor <--DistinctFrom--> Patient 