You may need to introduce additional Entities that aren't explicitly called out in the Business Rules to make your model work.  This is a normal part of translating requirements into a model.  

Typically in this domain you would have an Entity that represents an Episode-of-Care.  In a Hospital, this might be called an Admission, but more generally it's the thing to which contains a set of Diagnoses, including a Primary Diagnosis, a set of Orders, Procedures, etc.