Which resource defines the characteristics of a message that can be shared between systems, including the type of event that initiates the message, the content to be transmitted and what response(s), if any, are permitted?

• A. StructureDefinition
• B. SearchParameter
• C. CodeSystem
• D. MessageDefinition

Answer: (D)

In the FHIR messaging framework, the blueprint for how a message is exchanged between systems is defined by a resource that specifies the trigger event, the payload to be transmitted, and the allowed responses. This resource describes exactly what starts a message, what content should be carried, and what kind of acknowledgment or reply is permitted, providing a contract that both sending and receiving systems follow. That makes it the best fit for what the question asks.

This resource explicitly defines the event that initiates the message (the trigger), the content to be transmitted (the payload), and the response rules (whether a response is required and what form it may take). It’s distinct from others: StructureDefinition defines constraints on resource structures for profiles, SearchParameter defines how to search resources, and CodeSystem defines a set of codes.