What term is used for the published group that defines how an extension is structured and constrained?

• A. StructureDefinition
• B. Resource
• C. ValueSet
• D. CodeSystem

Answer: (A)

In FHIR, the published artifact that defines how an extension is structured and constrained is StructureDefinition. This resource lays out the exact shape of the extension—its elements, data types, cardinalities, and any rules or constraints—along with the context or place where the extension can be used (the parent resource or element). For an extension, the StructureDefinition specifies the extension’s URL, the type (typically Extension), and how its value is represented, ensuring that different systems interpret the extension consistently.

ValueSet and CodeSystem define allowable codes or vocabularies used within some elements, but they do not describe the structure of an extension. A Resource is the general base type for FHIR data and isn’t the defining artifact for an extension’s schema.