The extension definition is made available as which artifact?

• A. StructureDefinition
• B. ElementDefinition
• C. Resource
• D. Profile

Answer: (A)

An extension definition in FHIR is published as a StructureDefinition resource. The StructureDefinition carries the extension’s URL and metadata and defines the extension’s shape through ElementDefinitions inside it. In other words, the individual fields of the extension are described by ElementDefinition entries within that StructureDefinition. A Profile is a set of constraints applied to a resource or extension and does not itself define the extension. An ElementDefinition by itself is only a component inside a StructureDefinition, not the standalone artifact that defines the extension. A plain Resource is a base FHIR resource type, not the extension’s definitional artifact. So the extension definition is made available as a StructureDefinition.