Creating a call from a Medical Event in CRM iRep displays the default attendees from the Account rather than the Medical Event Attendees.
The Call page layout is missing the Medical_Event_vod__c field.
If the Medical Event has an associated account, then the Medical Event page layout contains Account_vod__c. The Call layout does not display Medical_Event_vod__c. The default call attendees do not come from the Medical Event attendees.
The default Call attendees that display come from the account affiliated with the Medical Events account. These are the same attendees that display if the user navigates to the Medical Event account and creates a Call.
CRM Help Documentation: Calls and Medical Events